(Slip Opinion)              OCTOBER TERM, 2009                                       1

                                       Syllabus

         NOTE: Where it is feasible, a syllabus (headnote) will be released, as is
       being done in connection with this case, at the time the opinion is issued.
       The syllabus constitutes no part of the opinion of the Court but has been
       prepared by the Reporter of Decisions for the convenience of the reader.
       See United States v. Detroit Timber & Lumber Co., 

, 337.


SUPREME COURT OF THE UNITED STATES

                                       Syllabus

             KRUPSKI v. COSTA CROCIERE S. P. A.

CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR
                THE ELEVENTH CIRCUIT

       No. 09–337.      Argued April 21, 2010—Decided June 7, 2010
Petitioner Krupski sought compensation for injuries she suffered on a
  cruise ship. Her passenger ticket, which was issued by Costa Cruise
  Lines, identified respondent Costa Crociere S. p. A. as the carrier; re
  quired an injured party to submit to the carrier or its agent written
  notice of a claim; required any lawsuit to be filed within one year of
  the injury; and designated a specific Federal District Court as the ex
  clusive forum for lawsuits such as Krupski’s. The front of the ticket
  listed Costa Cruise’s Florida address and made references to “Costa
  Cruises.” After Krupski’s attorney notified Costa Cruise of her
  claims but did not reach a settlement, Krupski filed a diversity negli
  gence action against Costa Cruise. Over the next several months—
  after the limitations period had expired—Costa Cruise brought Costa
  Crociere’s existence to Krupski’s attention three times, including in
  its motion for summary judgment, in which it stated that Costa Cro
  ciere was the proper defendant. Krupski responded and moved to
  amend her complaint to add Costa Crociere as a defendant. The Dis
  trict Court denied Costa Cruise’s summary judgment motion without
  prejudice and granted Krupski leave to amend. After she served
  Costa Crociere with an amended complaint, the court dismissed
  Costa Cruise from the case. Thereafter, Costa Crociere—represented
  by the same counsel as Costa Cruise—moved to dismiss, contending
  that the amended complaint did not satisfy the requirements of Fed
  eral Rule of Civil Procedure 15(c), which governs when an amended
  pleading “relates back” to the date of a timely filed original pleading
  and is thus timely even though it was filed outside an applicable limi
  tations period. The Rule requires, inter alia, that within the Rule
  4(m) 120-day period for service after a complaint is filed, the newly
  named defendant “knew or should have known that the action would
2                KRUPSKI v. COSTA CROCIERE S. P. A.

                                 Syllabus

    have been brought against it, but for a mistake concerning the proper
    party’s identity.” Rule 15(c)(1)(C)(ii). The District Court found this
    condition fatal to Krupski’s attempt to relate back. It concluded that
    she had not made a mistake about the proper party’s identity be
    cause, although Costa Cruise had disclosed Costa Crociere’s role in
    several court filings, she nonetheless delayed for months filing an
    amended complaint. The Eleventh Circuit affirmed, finding that
    Krupski either knew or should have known of Costa Crociere’s iden
    tity as a potential party because she furnished the ticket identifying
    it to her counsel well before the limitations period ended. It was
    therefore appropriate to treat her as having chosen to sue one poten
    tial party over another. Additionally, the court held that relation
    back was not appropriate because of Krupski’s undue delay in seek
    ing to amend the complaint.
Held: Relation back under Rule 15(c)(1)(C) depends on what the party
 to be added knew or should have known, not on the amending party’s
 knowledge or timeliness in seeking to amend the pleading. Pp. 7–18.
    (a) The Rule’s text does not support the Eleventh Circuit’s decision
 to rely on the plaintiff’s knowledge in denying relation back. The
 question under Rule 15(c)(1)(C)(ii) is not whether Krupski knew or
 should have known Costa Crociere’s identity as the proper defendant,
 but whether Costa Crociere knew or should have known during the
 Rule 4(m) period that it would have been named as the defendant but
 for an error. The plaintiff’s information is relevant only if it bears on
 the defendant’s understanding of whether the plaintiff made a mis
 take regarding the proper party’s identity. It would be error to con
 flate knowledge of a party’s existence with the absence of mistake.
 That a plaintiff knows of a party’s existence does not preclude her
 from making a mistake with respect to that party’s identity. Making
 a deliberate choice to sue one party over another while understand
 ing the factual and legal differences between the two parties may be
 the antithesis of making a mistake, but that does not mean that any
 time a plaintiff is aware of the existence of two parties and chooses to
 sue the wrong one, the proper defendant could reasonably believe
 that the plaintiff made no mistake. A plaintiff might know that the
 prospective defendant exists but nonetheless choose to sue a different
 defendant based on a misunderstanding about the proper party’s
 identity. That kind of deliberate but mistaken choice should not fore
 close a finding that Rule 15(c)(1)(C)(ii) has been satisfied. This read
 ing is consistent with relation back’s purpose of balancing the defen
 dant’s interests protected by the statute of limitations with the
 preference of the Federal Rules of Civil Procedure in general, and
 Rule 15 in particular, for resolving disputes on their merits. It is also
 consistent with the history of Rule 15(c)(1)(C). And it is not fore
                   Cite as: 560 U. S. ____ (2010)                      3

                              Syllabus

closed by Nelson v. Adams USA, Inc., 

. Pp. 7–13.
   (b) The Eleventh Circuit also erred in ruling that Krupski’s undue
delay in seeking to file, and in eventually filing, an amended com
plaint justified its denial of relation back under Rule 15(c)(1)(C). The
Rule plainly sets forth an exclusive list of requirements for relation
back, and the plaintiff’s diligence is not among them. Moreover, it
mandates relation back once its requirements are satisfied; it does
not leave that decision to the district court’s equitable discretion. Its
mandatory nature is particularly striking in contrast to the inquiry
under Rule 15(a), which gives a district court discretion to decide
whether to grant a motion to amend a pleading before trial. See
Foman v. Davis, 

, 182. Rule 15(c)(1)(C) permits a court
to examine a plaintiff’s conduct during the Rule 4(m) period, but only
to the extent the plaintiff’s postfiling conduct informs the prospective
defendant’s understanding of whether the plaintiff initially made a
“mistake concerning the proper party’s identity.” The plaintiff’s post
filing conduct is otherwise immaterial to the relation-back question.
Pp. 13–15.
   (c) Under these principles, the courts below erred in denying rela
tion back. Because the original complaint (of which Costa Crociere
had constructive notice) made clear that Krupski meant to sue the
company that “owned, operated, managed, supervised and controlled”
the ship on which she was injured and also indicated (mistakenly)
that Costa Cruise performed those roles, Costa Crociere should have
known that it avoided suit within the limitations period only because
of Krupski’s misunderstanding about which “Costa” entity was in
charge of the ship—clearly a “mistake concerning the proper party’s
identity.” That Krupski may have known the ticket’s contents does
not foreclose the possibility that she nonetheless misunderstood cru
cial facts regarding the two companies’ identities. Especially because
the face of the complaint plainly indicated such a misunderstanding,
respondent’s contention that it was entitled to think she made no
mistake is not persuasive. Moreover, respondent has articulated no
strategy that it could reasonably have thought Krupski was pursuing
in suing a defendant that was legally unable to provide relief. Noth
ing in Krupski’s conduct during the Rule 4(m) period suggests that
she failed to name Costa Crociere because of anything other than a
mistake. The interrelationship between Costa Cruise and Costa Cro
ciere and their similar names heighten the expectation that Costa
Crociere should suspect a mistake when Costa Cruise is named in a
complaint actually describing Costa Crociere’s activities. In addition,
Costa Crociere’s own actions contributed to passenger confusion over
“the proper party”: The front of the ticket advertises that “Costa
Cruises” has achieved a certification of quality without clarifying
4                KRUPSKI v. COSTA CROCIERE S. P. A.

                                 Syllabus

    which “Costa” company is meant. And as shown in similar lawsuits,
    Costa Crociere is evidently well aware that the difference between it
    and Costa Cruise can be confusing for passengers. Pp. 15–18.
330 Fed. Appx. 892, reversed and remanded.

  SOTOMAYOR, J., delivered the opinion of the Court, in which ROBERTS,
C. J., and STEVENS, KENNEDY, THOMAS, GINSBURG, BREYER, and ALITO,
JJ., joined. SCALIA, J., filed an opinion concurring in part and concur
ring in the judgment.
                        Cite as: 560 U. S. ____ (2010)                              1

                             Opinion of the Court

     NOTICE: This opinion is subject to formal revision before publication in the
     preliminary print of the United States Reports. Readers are requested to
     notify the Reporter of Decisions, Supreme Court of the United States, Wash
     ington, D. C. 20543, of any typographical or other formal errors, in order
     that corrections may be made before the preliminary print goes to press.


SUPREME COURT OF THE UNITED STATES
                                   _________________

                                   No. 09–337
                                   _________________


       WANDA KRUPSKI, PETITIONER v. COSTA 

               CROCIERE S. P. A. 

 ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF 

          APPEALS FOR THE ELEVENTH CIRCUIT

                                 [June 7, 2010]


  JUSTICE SOTOMAYOR delivered the opinion of the Court.
  Rule 15(c) of the Federal Rules of Civil Procedure gov
erns when an amended pleading “relates back” to the date
of a timely filed original pleading and is thus itself timely
even though it was filed outside an applicable statute of
limitations. Where an amended pleading changes a party
or a party’s name, the Rule requires, among other things,
that “the party to be brought in by amendment . . . knew
or should have known that the action would have been
brought against it, but for a mistake concerning the proper
party’s identity.” Rule 15(c)(1)(C). In this case, the Court
of Appeals held that Rule 15(c) was not satisfied because
the plaintiff knew or should have known of the proper
defendant before filing her original complaint. The court
also held that relation back was not appropriate because
the plaintiff had unduly delayed in seeking to amend. We
hold that relation back under Rule 15(c)(1)(C) depends on
what the party to be added knew or should have known,
not on the amending party’s knowledge or its timeliness in
seeking to amend the pleading. Accordingly, we reverse
the judgment of the Court of Appeals.
2            KRUPSKI v. COSTA CROCIERE S. P. A. 


                      Opinion of the Court 


                              I

   On February 21, 2007, petitioner, Wanda Krupski,
tripped over a cable and fractured her femur while she
was on board the cruise ship Costa Magica. Upon her
return home, she acquired counsel and began the process
of seeking compensation for her injuries. Krupski’s pas
senger ticket—which explained that it was the sole con
tract between each passenger and the carrier, App. to Pet.
for Cert. 37a—included a variety of requirements for
obtaining damages for an injury suffered on board one of
the carrier’s ships. The ticket identified the carrier as
    “Costa Crociere S. p. A., an Italian corporation, and all
    Vessels and other ships owned, chartered, operated,
    marketed or provided by Costa Crociere, S. p. A., and
    all officers, staff members, crew members, independ
    ent contractors, medical providers, concessionaires, pi
    lots, suppliers, agents and assigns onboard said Ves
    sels, and the manufacturers of said Vessels and all
    their component parts.” 

The ticket required an injured party to submit “written
notice of the claim with full particulars . . . to the carrier
or its duly authorized agent within 185 days after the date
of injury.” 

The ticket further required any
lawsuit to be “filed within one year after the date of in
jury” and to be “served upon the carrier within 120 days
after filing.” 

from voyages depart
ing from or returning to a United States port in which the
amount in controversy exceeded $75,000, the ticket desig
nated the United States District Court for the Southern
District of Florida in Broward County, Florida, as the
exclusive forum for a lawsuit. 

The ticket
extended the “defenses, limitations and exceptions . . . that
may be invoked by the CARRIER” to “all persons who may
act on behalf of the CARRIER or on whose behalf the
CARRIER may act,” including “the CARRIER’s parents,
                 Cite as: 560 U. S. ____ (2010)            3

                     Opinion of the Court

subsidiaries, affiliates, successors, assigns, representa
tives, agents, employees, servants, concessionaires and
contractors” as well as “Costa Cruise Lines N. V.,” identi
fied as the “sales and marketing agent for the CARRIER
and the issuer of this Passage Ticket Contract.” 

The front of the ticket listed Costa Cruise Lines’
address in Florida and stated that an entity called “Costa
Cruises” was “the first cruise company in the world” to
obtain a certain certification of quality. 

   On July 2, 2007, Krupski’s counsel notified Costa Cruise
Lines of Krupski’s claims. App. 69–70. On July 9, 2007,
the claims administrator for Costa Cruise requested addi
tional information from Krupski “[i]n order to facilitate
our future attempts to achieve a pre-litigation settlement.”
App. to Pet. for Cert. 23a–24a. The parties were unable to
reach a settlement, however, and on February 1, 2008—
three weeks before the 1-year limitations period expired—
Krupski filed a negligence action against Costa Cruise,
invoking the diversity jurisdiction of the Federal District
Court for the Southern District of Florida. The complaint
alleged that Costa Cruise “owned, operated, managed,
supervised and controlled” the ship on which Krupski had
injured herself; that Costa Cruise had extended to its
passengers an invitation to enter onto the ship; and that
Costa Cruise owed Krupski a duty of care, which it
breached by failing to take steps that would have pre
vented her accident. App. 23–26. The complaint further
stated that venue was proper under the passenger ticket’s
forum selection clause and averred that, by the July 2007
notice of her claims, Krupski had complied with the
ticket’s presuit requirements. 

Krupski served
Costa Cruise on February 4, 2008.
   Over the next several months—after the limitations
period had expired—Costa Cruise brought Costa Croci
ere’s existence to Krupski’s attention three times. First,
on February 25, 2008, Costa Cruise filed its answer, as
4           KRUPSKI v. COSTA CROCIERE S. P. A.

                     Opinion of the Court

serting that it was not the proper defendant, as it was
merely the North American sales and marketing agent for
Costa Crociere, which was the actual carrier and vessel
operator. 

Second, on March 20, 2008, Costa
Cruise listed Costa Crociere as an interested party in its
corporate disclosure statement. App. to Pet. for Cert. 20a.
Finally, on May 6, 2008, Costa Cruise moved for summary
judgment, again stating that Costa Crociere was the
proper defendant. App. 5, 33–38.
   On June 13, 2008, Krupski responded to Costa Cruise’s
motion for summary judgment, arguing for limited discov
ery to determine whether Costa Cruise should be dis
missed. According to Krupski, the following sources of
information led her to believe Costa Cruise was the re
sponsible party: The travel documents prominently identi
fied Costa Cruise and gave its Florida address; Costa
Cruise’s Web site listed Costa Cruise in Florida as the
United States office for the Italian company Costa Croci
ere; and the Web site of the Florida Department of State
listed Costa Cruise as the only “Costa” company registered
to do business in that State. 

56–59. Krup
ski also observed that Costa Cruise’s claims administrator
had responded to her claims notification without indicat
ing that Costa Cruise was not a responsible party. 

With her response, Krupski simultaneously moved to
amend her complaint to add Costa Crociere as a defen
dant. 

52–54.
   On July 2, 2008, after oral argument, the District Court
denied Costa Cruise’s motion for summary judgment
without prejudice and granted Krupski leave to amend,
ordering that Krupski effect proper service on Costa Cro
ciere by September 16, 2008. 

Complying
with the court’s deadline, Krupski filed an amended com
plaint on July 11, 2008, and served Costa Crociere on
August 21, 2008. 

88–89. On that same date,
the District Court issued an order dismissing Costa Cruise
                 Cite as: 560 U. S. ____ (2010)            5

                     Opinion of the Court

from the case pursuant to the parties’ joint stipulation,
Krupski apparently having concluded that Costa Cruise
was correct that it bore no responsibility for her injuries.

  Shortly thereafter, Costa Crociere—represented by the
same counsel who had represented Costa Cruise, compare

with 

to dismiss, contending
that the amended complaint did not relate back under
Rule 15(c) and was therefore untimely. The District Court
agreed. App. to Pet. for Cert. 8a–22a. Rule 15(c), the
court explained, imposes three requirements before an
amended complaint against a newly named defendant can
relate back to the original complaint. First, the claim
against the newly named defendant must have arisen “out
of the conduct, transaction, or occurrence set out—or
attempted to be set out—in the original pleading.” Fed.
Rules Civ. Proc. 15(c)(1)(B), (C). Second, “within the
period provided by Rule 4(m) for serving the summons and
complaint” (which is ordinarily 120 days from when the
complaint is filed, see Rule 4(m)), the newly named defen
dant must have “received such notice of the action that it
will not be prejudiced in defending on the merits.” Rule
15(c)(1)(C)(i). Finally, the plaintiff must show that, within
the Rule 4(m) period, the newly named defendant “knew
or should have known that the action would have been
brought against it, but for a mistake concerning the proper
party’s identity.” Rule 15(c)(1)(C)(ii).
  The first two conditions posed no problem, the court
explained: The claim against Costa Crociere clearly in
volved the same occurrence as the original claim against
Costa Cruise, and Costa Crociere had constructive notice
of the action and had not shown that any unfair prejudice
would result from relation back. App. to Pet. for Cert.
14a–18a. But the court found the third condition fatal to
Krupski’s attempt to relate back, concluding that Krupski
had not made a mistake concerning the identity of the
6             KRUPSKI v. COSTA CROCIERE S. P. A.

                        Opinion of the Court

proper party. 

Relying on Eleventh Cir
cuit precedent, the court explained that the word “mis
take” should not be construed to encompass a deliberate
decision not to sue a party whose identity the plaintiff
knew before the statute of limitations had run. Because
Costa Cruise informed Krupski that Costa Crociere was
the proper defendant in its answer, corporate disclosure
statement, and motion for summary judgment, and yet
Krupski delayed for months in moving to amend and then
in filing an amended complaint, the court concluded that
Krupski knew of the proper defendant and made no
mistake.
   The Eleventh Circuit affirmed in an unpublished per
curiam opinion. Krupski v. Costa Cruise Lines, N. V.,
LLC, 330 Fed. Appx. 892 (2009). Rather than relying on
the information contained in Costa Cruise’s filings, all of
which were made after the statute of limitations had
expired, as evidence that Krupski did not make a mistake,
the Court of Appeals noted that the relevant information
was located within Krupski’s passenger ticket, which she
had furnished to her counsel well before the end of the
limitations period. Because the ticket clearly identified
Costa Crociere as the carrier, the court stated, Krupski
either knew or should have known of Costa Crociere’s
identity as a potential party.1 It was therefore appropri
ate to treat Krupski as having chosen to sue one potential
party over another. Alternatively, even assuming that she
first learned of Costa Crociere’s identity as the correct
party from Costa Cruise’s answer, the Court of Appeals
——————
  1 The Court of Appeals stated that it was “imput[ing]” knowledge to

Krupski. 330 Fed. Appx., at 895. Petitioner uses the terms “imputed
knowledge” and “constructive knowledge” interchangeably in her brief,
while respondent addresses only actual knowledge. Because we reject
the Court of Appeals’ focus on the plaintiff’s knowledge in the first
instance, see infra, at 8–13, the distinction among these types of
knowledge is not relevant to our resolution of this case.
                     Cite as: 560 U. S. ____ (2010)                   7

                         Opinion of the Court

observed that Krupski waited 133 days from the time she
filed her original complaint to seek leave to amend and did
not file an amended complaint for another month after
that. In light of this delay, the Court of Appeals concluded
that the District Court did not abuse its discretion in
denying relation back.
   We granted certiorari to resolve tension among the
Circuits over the breadth of Rule 15(c)(1)(C)(ii),2 558 U. S.
___ (2010), and we now reverse.
                            II
  Under the Federal Rules of Civil Procedure, an amend
ment to a pleading relates back to the date of the original
pleading when:
       “(A) the law that provides the applicable statute of
     limitations allows relation back;
       “(B) the amendment asserts a claim or defense that
     arose out of the conduct, transaction, or occurrence set
     out—or attempted to be set out—in the original plead
——————
  2 See, e.g., Krupski v. Costa Cruise Lines, N. V., LLC, 330 Fed. Appx.

892, 895 (CA11 2009) (per curiam) (case below); Rendall-Speranza v.
Nassim, 

, 918 (CADC 1997) (provision does not authorize
relation back where plaintiff “was fully aware of the potential defen
dant’s identity but not of its responsibility for the harm alleged”);
Cornwell v. Robinson, 

, 705 (CA2 1994) (no relation back
where plaintiff knew the identities of the responsible defendants and
failed to name them); Goodman v. Praxair, Inc., 

, 469–470
(CA4 2007) (en banc) (rejecting argument that plaintiff’s knowledge of
proper corporate defendant’s existence and name meant that no mis
take had been made); Arthur v. Maersk, Inc., 

, 208 (CA3
2006) (“A ‘mistake’ is no less a ‘mistake’ when it flows from lack of
knowledge as opposed to inaccurate description”); Leonard v. Parry, 

, 28–29 (CA1 2000) (plaintiff’s knowledge of proper defendant’s
identity was not relevant to whether she made a “ ‘mistake concerning
the identity of the proper party’ ”). We express no view on whether
these decisions may be reconciled with each other in light of their
specific facts and the interpretation of Rule 15(c)(1)(C)(ii) we adopt
today.
8             KRUPSKI v. COSTA CROCIERE S. P. A.

                      Opinion of the Court

    ing; or
      “(C) the amendment changes the party or the nam
    ing of the party against whom a claim is asserted, if
    Rule 15(c)(1)(B) is satisfied and if, within the period
    provided by Rule 4(m) for serving the summons and
    complaint, the party to be brought in by amendment:
      “(i) received such notice of the action that it will not
    be prejudiced in defending on the merits; and
      “(ii) knew or should have known that the action
    would have been brought against it, but for a mistake
    concerning the proper party’s identity.” Rule 15(c)(1).
In our view, neither of the Court of Appeals’ reasons for
denying relation back under Rule 15(c)(1)(C)(ii) finds
support in the text of the Rule. We consider each reason
in turn.
                              A
  The Court of Appeals first decided that Krupski either
knew or should have known of the proper party’s identity
and thus determined that she had made a deliberate
choice instead of a mistake in not naming Costa Crociere
as a party in her original pleading. 330 Fed. Appx., at
895. By focusing on Krupski’s knowledge, the Court of
Appeals chose the wrong starting point. The question
under Rule 15(c)(1)(C)(ii) is not whether Krupski knew or
should have known the identity of Costa Crociere as the
proper defendant, but whether Costa Crociere knew or
should have known that it would have been named as a
defendant but for an error. Rule 15(c)(1)(C)(ii) asks what
the prospective defendant knew or should have known
during the Rule 4(m) period, not what the plaintiff knew
or should have known at the time of filing her original
                      Cite as: 560 U. S. ____ (2010)                       9

                           Opinion of the Court

complaint.3
   Information in the plaintiff’s possession is relevant only
if it bears on the defendant’s understanding of whether
the plaintiff made a mistake regarding the proper party’s
identity. For purposes of that inquiry, it would be error to
conflate knowledge of a party’s existence with the absence
of mistake. A mistake is “[a]n error, misconception, or
misunderstanding; an erroneous belief.” Black’s Law
Dictionary 1092 (9th ed. 2009); see also Webster’s Third
New International Dictionary 1446 (2002) (defining “mis
take” as “a misunderstanding of the meaning or implica
tion of something”; “a wrong action or statement proceed
ing from faulty judgment, inadequate knowledge, or
inattention”; “an erroneous belief”; or “a state of mind not
in accordance with the facts”). That a plaintiff knows of a
party’s existence does not preclude her from making a
mistake with respect to that party’s identity. A plaintiff
may know that a prospective defendant—call him party
A—exists, while erroneously believing him to have the
status of party B. Similarly, a plaintiff may know gener
ally what party A does while misunderstanding the roles
that party A and party B played in the “conduct, transac
tion, or occurrence” giving rise to her claim. If the plaintiff
sues party B instead of party A under these circum
stances, she has made a “mistake concerning the proper
party’s identity” notwithstanding her knowledge of the
existence of both parties. The only question under Rule
——————
   3 Rule 15(c)(1)(C) speaks generally of an amendment to a “pleading”

that changes “the party against whom a claim is asserted,” and it
therefore is not limited to the circumstance of a plaintiff filing an
amended complaint seeking to bring in a new defendant. Nevertheless,
because the latter is the “typical case” of Rule 15(c)(1)(C)’s applicability,
see 3 Moore’s Federal Practice §15.19[2] (3d ed. 2009), we use this
circumstance as a shorthand throughout this opinion. See also 

Committee’s 1966 Notes on Fed. Rule Civ.
Proc. 15, 

. App., pp. 122–123 (hereinafter Advisory Commit
tee’s 1966 Notes).
10           KRUPSKI v. COSTA CROCIERE S. P. A.

                     Opinion of the Court

15(c)(1)(C)(ii), then, is whether party A knew or should
have known that, absent some mistake, the action would
have been brought against him.
   Respondent urges that the key issue under Rule
15(c)(1)(C)(ii) is whether the plaintiff made a deliberate
choice to sue one party over another. Brief for Respondent
11–16. We agree that making a deliberate choice to sue
one party instead of another while fully understanding the
factual and legal differences between the two parties is the
antithesis of making a mistake concerning the proper
party’s identity. We disagree, however, with respondent’s
position that any time a plaintiff is aware of the existence
of two parties and chooses to sue the wrong one, the
proper defendant could reasonably believe that the plain
tiff made no mistake. The reasonableness of the mistake
is not itself at issue. As noted, a plaintiff might know that
the prospective defendant exists but nonetheless harbor a
misunderstanding about his status or role in the events
giving rise to the claim at issue, and she may mistakenly
choose to sue a different defendant based on that misim
pression. That kind of deliberate but mistaken choice does
not foreclose a finding that Rule 15(c)(1)(C)(ii) has been
satisfied.
   This reading is consistent with the purpose of relation
back: to balance the interests of the defendant protected
by the statute of limitations with the preference expressed
in the Federal Rules of Civil Procedure in general, and
Rule 15 in particular, for resolving disputes on their mer
its. See, e.g., Advisory Committee’s 1966 Notes 122; 3
Moore’s Federal Practice §§15.02[1], 15.19[3][a] (3d ed.
2009). A prospective defendant who legitimately believed
that the limitations period had passed without any at
tempt to sue him has a strong interest in repose. But
repose would be a windfall for a prospective defendant
who understood, or who should have understood, that he
escaped suit during the limitations period only because
                 Cite as: 560 U. S. ____ (2010)          11

                     Opinion of the Court

the plaintiff misunderstood a crucial fact about his iden
tity. Because a plaintiff’s knowledge of the existence of a
party does not foreclose the possibility that she has made
a mistake of identity about which that party should have
been aware, such knowledge does not support that party’s
interest in repose.
   Our reading is also consistent with the history of Rule
15(c)(1)(C). That provision was added in 1966 to respond
to a recurring problem in suits against the Federal Gov
ernment, particularly in the Social Security context.
Advisory Committee’s 1966 Notes 122. Individuals who
had filed timely lawsuits challenging the administrative
denial of benefits often failed to name the party identified
in the statute as the proper defendant—the current Secre
tary of what was then the Department of Health, Educa
tion, and Welfare—and named instead the United States;
the Department of Health, Education, and Welfare itself;
the nonexistent “Federal Security Administration”; or a
Secretary who had recently retired from office. 

the plaintiffs discovered their mistakes, the
statute of limitations in many cases had expired, and the
district courts denied the plaintiffs leave to amend on the
ground that the amended complaints would not relate
back. Rule 15(c) was therefore “amplified to provide a
general solution” to this problem. 

that the Social Security litigants knew or reasonably
should have known the identity of the proper defendant
either because of documents in their administrative cases
or by dint of the statute setting forth the filing require
ments. See 

. §405(g) (1958 ed., Supp. III).
Nonetheless, the Advisory Committee clearly meant their
filings to qualify as mistakes under the Rule.
   Respondent suggests that our decision in Nelson v.
Adams USA, Inc., 

(2000), forecloses the
reading of Rule 15(c)(1)(C)(ii) we adopt today. We dis
agree. In that case, Adams USA, Inc. (Adams), had ob
12             KRUPSKI v. COSTA CROCIERE S. P. A.

                          Opinion of the Court

tained an award of attorney’s fees against the corporation
of which Donald Nelson was the president and sole share
holder. After Adams became concerned that the corpora
tion did not have sufficient funds to pay the award, Adams
sought to amend its pleading to add Nelson as a party and
simultaneously moved to amend the judgment to hold
Nelson responsible. The District Court granted both
motions, and the Court of Appeals affirmed. We reversed,
holding that the requirements of due process, as codified
in Rules 12 and 15 of the Federal Rules of Civil Procedure,
demand that an added party have the opportunity to
respond before judgment is entered against him. 

In a footnote explaining that relation back does
not deny the added party an opportunity to respond to the
amended pleading, we noted that the case did not arise
under the “mistake clause” of Rule 15(c):4 “Respondent
Adams made no such mistake. It knew of Nelson’s role
and existence and, until it moved to amend its pleading,
chose to assert its claim for costs and fees only against
[Nelson’s company].” 

n. 1.
   Contrary to respondent’s claim, Nelson does not suggest
that Rule 15(c)(1)(C)(ii) cannot be satisfied if a plaintiff
knew of the prospective defendant’s existence at the time
she filed her original complaint. In that case, there was
nothing in the initial pleading suggesting that Nelson was
an intended party, while there was evidence in the record
(of which Nelson was aware) that Adams sought to add
him only after learning that the company would not be
able to satisfy the judgment. 

This evi
——————
  4 The “mistake clause” at the time we decided Nelson was set forth in

Rule 

, n. 1; 528 F. R. D. 525, 529 (1991). Rule
15(c) was renumbered in 2007 without substantive change “as part of
the general restyling of the Civil Rules,” at which time it received its
current placement in Rule 15(c)(1)(C)(ii). Advisory Committee’s 2007
Notes on Fed. Rule Civ. Proc. 15, 

. App., p. 37 (2006 ed.,
Supp. II).
                  Cite as: 560 U. S. ____ (2010)            13

                      Opinion of the Court

dence countered any implication that Adams had origi
nally failed to name Nelson because of any “mistake con
cerning the proper party’s identity,” and instead suggested
that Adams decided to name Nelson only after the fact in
an attempt to ensure that the fee award would be paid.
The footnote merely observes that Adams had originally
been under no misimpression about the function Nelson
played in the underlying dispute. We said, after all, that
Adams knew of Nelson’s “role” as well as his existence.

n. 1. Read in context, the footnote in Nelson is
entirely consistent with our understanding of the Rule:
When the original complaint and the plaintiff’s conduct
compel the conclusion that the failure to name the pro
spective defendant in the original complaint was the result
of a fully informed decision as opposed to a mistake con
cerning the proper defendant’s identity, the requirements
of Rule 15(c)(1)(C)(ii) are not met. This conclusion is in
keeping with our rejection today of the Court of Appeals’
reliance on the plaintiff’s knowledge to deny relation back.
                               B
  The Court of Appeals offered a second reason why Krup
ski’s amended complaint did not relate back: Krupski had
unduly delayed in seeking to file, and in eventually filing,
an amended complaint. 330 Fed. Appx., at 895. The
Court of Appeals offered no support for its view that a
plaintiff’s dilatory conduct can justify the denial of relation
back under Rule 15(c)(1)(C), and we find none. The Rule
plainly sets forth an exclusive list of requirements for
relation back, and the amending party’s diligence is not
among them. Moreover, the Rule mandates relation back
once the Rule’s requirements are satisfied; it does not
leave the decision whether to grant relation back to the
district court’s equitable discretion. See Rule 15(c)(1) (“An
amendment . . . relates back . . . when” the three listed
requirements are met (emphasis added)).
14          KRUPSKI v. COSTA CROCIERE S. P. A.

                     Opinion of the Court

   The mandatory nature of the inquiry for relation back
under Rule 15(c) is particularly striking in contrast to the
inquiry under Rule 15(a), which sets forth the circum
stances in which a party may amend its pleading before
trial. By its terms, Rule 15(a) gives discretion to the
district court in deciding whether to grant a motion to
amend a pleading to add a party or a claim. Following an
initial period after filing a pleading during which a party
may amend once “as a matter of course,” “a party may
amend its pleading only with the opposing party’s written
consent or the court’s leave,” which the court “should
freely give . . . when justice so requires.” Rules 15(a)(1)–
(2). We have previously explained that a court may con
sider a movant’s “undue delay” or “dilatory motive” in
deciding whether to grant leave to amend under Rule
15(a). Foman v. Davis, 

, 182 (1962). As the
contrast between Rule 15(a) and Rule 15(c) makes clear,
however, the speed with which a plaintiff moves to amend
her complaint or files an amended complaint after obtain
ing leave to do so has no bearing on whether the amended
complaint relates back. Cf. 6A C. Wright, A. Miller, & M.
Kane, Federal Practice and Procedure §1498, pp. 142–143,
and nn. 49–50 (2d ed. 1990 and Supp. 2010).
   Rule 15(c)(1)(C) does permit a court to examine a plain
tiff’s conduct during the Rule 4(m) period, but not in the
way or for the purpose respondent or the Court of Appeals
suggests. As we have explained, the question under Rule
15(c)(1)(C)(ii) is what the prospective defendant reasona
bly should have understood about the plaintiff’s intent in
filing the original complaint against the first defendant.
To the extent the plaintiff’s postfiling conduct informs the
prospective defendant’s understanding of whether the
plaintiff initially made a “mistake concerning the proper
party’s identity,” a court may consider the conduct. Cf.
Leonard v. Parry, 

, 29 (CA1 2000) (“[P]ost
filing events occasionally can shed light on the plaintiff’s
                     Cite as: 560 U. S. ____ (2010)                  15

                         Opinion of the Court

state of mind at an earlier time” and “can inform a defen
dant’s reasonable beliefs concerning whether her omission
from the original complaint represented a mistake (as
opposed to a conscious choice)”). The plaintiff’s postfiling
conduct is otherwise immaterial to the question whether
an amended complaint relates back.5
                             C
   Applying these principles to the facts of this case, we
think it clear that the courts below erred in denying rela
tion back under Rule 15(c)(1)(C)(ii). The District Court
held that Costa Crociere had “constructive notice” of
Krupski’s complaint within the Rule 4(m) period. App. to
Pet. for Cert. 15a–17a. Costa Crociere has not challenged
this finding. Because the complaint made clear that Krup
ski meant to sue the company that “owned, operated,
managed, supervised and controlled” the ship on which
she was injured, App. 23, and also indicated (mistakenly)
that Costa Cruise performed those roles, 

Costa Crociere should have known, within the Rule 4(m)
period, that it was not named as a defendant in that com
plaint only because of Krupski’s misunderstanding about
which “Costa” entity was in charge of the ship—clearly a
“mistake concerning the proper party’s identity.”
   Respondent contends that because the original com
plaint referred to the ticket’s forum requirement and
presuit claims notification procedure, Krupski was clearly
aware of the contents of the ticket, and because the ticket
identified Costa Crociere as the carrier and proper party
——————
  5 Similarly,we reject respondent’s suggestion that Rule 15(c) requires
a plaintiff to move to amend her complaint or to file and serve an
amended complaint within the Rule 4(m) period. Rule 15(c)(1)(C)(i)
simply requires that the prospective defendant has received sufficient
“notice of the action” within the Rule 4(m) period that he will not be
prejudiced in defending the case on the merits. The Advisory Commit
tee Notes to the 1966 Amendment clarify that “the notice need not be
formal.” Advisory Committee’s 1966 Notes 122.
16             KRUPSKI v. COSTA CROCIERE S. P. A.

                         Opinion of the Court

for a lawsuit, respondent was entitled to think that she
made a deliberate choice to sue Costa Cruise instead of
Costa Crociere. Brief for Respondent 13. As we have
explained, however, that Krupski may have known the
contents of the ticket does not foreclose the possibility that
she nonetheless misunderstood crucial facts regarding the
two companies’ identities. Especially because the face of
the complaint plainly indicated such a misunderstanding,
respondent’s contention is not persuasive. Moreover,
respondent has articulated no strategy that it could rea
sonably have thought Krupski was pursuing in suing a
defendant that was legally unable to provide relief.
  Respondent also argues that Krupski’s failure to move
to amend her complaint during the Rule 4(m) period
shows that she made no mistake in that period. 

But as discussed, any delay on Krupski’s part is
relevant only to the extent it may have informed Costa
Crociere’s understanding during the Rule 4(m) period of
whether she made a mistake originally. Krupski’s failure
to add Costa Crociere during the Rule 4(m) period is not
sufficient to make reasonable any belief that she had
made a deliberate and informed decision not to sue Costa
Crociere in the first instance.6 Nothing in Krupski’s con
duct during the Rule 4(m) period suggests that she failed
to name Costa Crociere because of anything other than a
mistake.
  It is also worth noting that Costa Cruise and Costa
——————
  6 The Court of Appeals concluded that Krupski was not diligent

merely because she did not seek leave to add Costa Crociere until 133
days after she filed her original complaint and did not actually file an
amended complaint for another a month after that. 330 Fed. Appx., at
895. It is not clear why Krupski should have been found dilatory for
not accepting at face value the unproven allegations in Costa Cruise’s
answer and corporate disclosure form. In fact, Krupski moved to
amend her complaint to add Costa Crociere within the time period
prescribed by the District Court’s scheduling order. See App. 3, 6–7;
Record, Doc. 23, p. 1.
                  Cite as: 560 U. S. ____ (2010)            17

                      Opinion of the Court

Crociere are related corporate entities with very similar
names; “crociera” even means “cruise” in Italian. Cassell’s
Italian Dictionary 137, 670 (1967). This interrelationship
and similarity heighten the expectation that Costa Croci
ere should suspect a mistake has been made when Costa
Cruise is named in a complaint that actually describes
Costa Crociere’s activities. Cf. Morel v. DaimlerChrysler
AG, 

, 27 (CA1 2009) (where complaint con
veyed plaintiffs’ attempt to sue automobile manufacturer
and erroneously named the manufacturer as Daimler-
Chrysler Corporation instead of the actual manufacturer,
a legally distinct but related entity named DaimlerChrys
ler AG, the latter should have realized it had not been
named because of plaintiffs’ mistake); Goodman v. Prax
air, Inc., 

, 473–475 (CA4 2007) (en banc)
(where complaint named parent company Praxair, Inc.,
but described status of subsidiary company Praxair Ser
vices, Inc., subsidiary company knew or should have
known it had not been named because of plaintiff’s mis
take). In addition, Costa Crociere’s own actions contrib
uted to passenger confusion over “the proper party” for a
lawsuit. The front of the ticket advertises that “Costa
Cruises” has achieved a certification of quality, App. to
Pet. for Cert. 25a, without clarifying whether “Costa
Cruises” is Costa Cruise Lines, Costa Crociere, or some
other related “Costa” company. Indeed, Costa Crociere is
evidently aware that the difference between Costa Cruise
and Costa Crociere can be confusing for cruise ship pas
sengers. See, e.g., Suppa v. Costa Crociere, S. p. A., No.
07–60526–CIV, 

, *1, (SD Fla., Dec. 4,
2007) (denying Costa Crociere’s motion to dismiss the
amended complaint where the original complaint had
named Costa Cruise as a defendant after “find[ing] it
simply inconceivable that Defendant Costa Crociere was
not on notice . . . that . . . but for the mistake in the origi
nal Complaint, Costa Crociere was the appropriate party
18          KRUPSKI v. COSTA CROCIERE S. P. A.

                     Opinion of the Court

to be named in the action”).
  In light of these facts, Costa Crociere should have
known that Krupski’s failure to name it as a defendant in
her original complaint was due to a mistake concerning
the proper party’s identity. We therefore reverse the
judgment of the Court of Appeals for the Eleventh Circuit
and remand the case for further proceedings consistent
with this opinion.
                                          It is so ordered.
                  Cite as: 560 U. S. ____ (2010)             1

                      Opinion of SCALIA, J.

SUPREME COURT OF THE UNITED STATES
                          _________________

                           No. 09–337
                          _________________


       WANDA KRUPSKI, PETITIONER v. COSTA 

               CROCIERE S. P. A. 

 ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF 

          APPEALS FOR THE ELEVENTH CIRCUIT

                         [June 7, 2010]


   JUSTICE SCALIA, concurring in part and concurring in
the judgment.
   I join the Court’s opinion except for its reliance, ante, at
10–11, 15, n. 5, on the Notes of the Advisory Committee as
establishing the meaning of Federal Rule of Civil Proce
dure 15(c)(1)(C). The Advisory Committee’s insights into
the proper interpretation of a Rule’s text are useful to the
same extent as any scholarly commentary. But the Com
mittee’s intentions have no effect on the Rule’s meaning.
Even assuming that we and the Congress that allowed the
Rule to take effect read and agreed with those intentions,
it is the text of the Rule that controls. Tome v. United
States, 

, 167–168 (1995) (SCALIA, J., concur
ring in part and concurring in judgment).