A motion to suppress is an attempt by the defense in a criminal case to keep evidence out of court. It gets the “pretrial” label where procedural rules require that the defense bring the motion before the trial stage.
A core principle of criminal procedure is that evidence that the authorities have illegally obtained is inadmissible at trial. That evidence could be:
a tangible item, like drugs discovered after the police illegally detained someone
a statement by the defendant. like a confession that the police coerced, or even
an eyewitness identification, like testimony from a witness who pointed a finger at the defendant only after hearing an officer say, “That’s the guy.”
The exclusionary rule isn’t designed to help judges and juries find the truth. The purpose of the rule—a rule that, by design, can result in a jury never hearing about relevant incriminating evidence—is to deter police agencies from violating people’s rights.
Advocates of the exclusionary rule contend that, without it, police officers would have little reason not to trample core rights while going after convictions. They say the rule is especially important in providing a check against abuse of the Fourth Amendment (which guarantees the right to be free from unreasonable searches and seizures).
Under the rules of many jurisdictions, defendants normally have to bring motions to suppress before trial. A defendant who doesn’t bring the motion before trial can risk waiving (losing) the opportunity to bring it at all.
It makes sense that a motion to suppress would have to occur before trial. If the defense wins the motion, the prosecution might not have a basis for even going to trial. That would likely be the situation where the charge against a defendant is illegal drug possession and the judge has tossed the evidence of the drugs.
Requiring that the defense bring the motion before the case gets in front of a jury can also avoid interruption of trial. Similarly, the pretrial requirement gives the prosecution a chance to appeal an order by a judge granting the defense's motion to suppress.
Motions to suppress often begin with written pleadings, then proceed to a hearing, where the judge listens to testimony and considers the sides’ oral arguments.
The prosecution frequently ends up with the “burden” of convincing the judge that the evidence should not be suppressed. For example, in a standard Fourth Amendment challenge, once the defense establishes that a police search occurred without a warrant, the prosecution has to establish a justification for the search.
If a judge grants a pretrial motion to suppress evidence, the prosecution has a weaker case. How much weaker the case is depends on the facts and the evidence, but it’s not unusual for the government to dismiss charges after losing such a motion.
It's important to note that the exclusionary rule has exceptions. A motion to suppress can fail where, for instance, a judge finds that the prosecution inevitably would have discovered the evidence. Under the “inevitable discovery doctrine,” evidence gotten through illegal means is admissible if the government shows the police would otherwise have legally acquired it.
Does the law in my state give me a better chance to win a suppression hearing than the federal constitution?
Does the exclusionary rule apply in other matters, like deportation proceedings and probation revocation hearings?
Can a defendant bring a motion to exclude evidence from a sentencing hearing?
When can the defense appeal an order denying a motion to suppress? Is it possible to plead guilty while reserving the right to appeal a judge’s denial of the motion?
What happens if you have two bases to ask for evidence to be suppressed but present only one of them?
