The Fourth Amendment to the federal constitution is the bedrock of U.S. search and seizure law. It guards people against “unreasonable searches and seizures” of their “persons, houses, papers, and effects.” It applies where law enforcement:

• searches or seizes property, or
• arrests someone.

The amendment takes its meaning from court opinions—most significantly, those of the United States Supreme Court. These opinions determine the kinds of searches and seizures that are unreasonable, and therefore unconstitutional.

The Meaning of “Search”

To establish that a search by the police violated the Constitution, one has to first show that the conduct actually amounted to a “search” within the meaning of the Fourth Amendment.

Law enforcement is considered to have carried out a Fourth-Amendment search if it, in an attempt to find something or gather information:

• breaches the defendant’s “reasonable expectation of privacy” or
• physically intrudes on the defendant’s property.

Expectation of Privacy

In order for the police to violate someone’s reasonable expectation of privacy, the person must, of course, reasonably expect privacy. Courts have found that you can’t reasonably expect that some kinds of evidence will be private.

Perhaps the best example of property not protected by a reasonable expectation of privacy is trash that’s been left out. If the police hunt through the garbage bin you’ve left for the sanitation company, they probably haven’t “searched” it within the meaning of the Fourth Amendment. In other words, whatever evidence they find will likely be fair game in court.

The police also aren’t considered to have carried out a search in most situations where they see, smell, or hear something while out in public. For example, an officer’s looking into a car window while standing on the sidewalk wouldn’t constitute a search.

Property Trespasses

Another way to carry out a Fourth-Amendment search is for law enforcement to physically intrude on someone’s property. An officer who intrudes in order to find evidence or information has carried out a search.

As an example, the U.S. Supreme Court once considered a case where the police attached a global-positioning-system (GPS) tracking device to the undercarriage of a vehicle and used it to monitor the vehicle as it moved around on public streets. The Court held that, regardless of whether the driver could reasonably expect his movements on public roads to remain private, the government had conducted a “search.” Putting the tracking device on the car was a physical intrusion. (United States v. Jones, 132 S. Ct. 945 (2012).)

Probable Cause

Let’s assume that the police have in fact conducted a Fourth-Amendment search, or that they arrested someone. The next question is usually whether they had probable cause.

The police have probable cause for an arrest when it is reasonably likely that:

• someone has violated the law, and
• the person to be arrested is that someone.

There’s probable cause to search a place if it’s reasonably likely that:

• the evidence to be found is connected with a crime, and
• the evidence will be found in the place the police will search.

The term “probable cause” doesn’t actually mean “more likely than not.” To illustrate, in one U.S. Supreme Court case, an officer found cocaine in the backseat of a car during a legitimate search. Three people had been in the car. The officer arrested all three. The front-seat passenger eventually confessed. The Court found that there was probable cause to arrest the front-seat passenger even though he wasn’t any more likely to own the cocaine than the other two people in the car. (Maryland v. Pringle, 540 U.S. 366 (2003).)

With or Without Warrants

Sometimes a judge or magistrate determines whether there’s probable cause before a search or seizure. Other times the police act, and a judge has to determine after the fact whether there was probable cause.

Warrants for Searches and Arrests

Some situations require that the police get a warrant before taking action. Searches that are especially invasive—like a search of a home—usually require search warrants. Arrest warrants aren’t mandated as often as search warrants. (Arrest warrants are, however, typically required when the police enter a home to make an arrest.)

Warrantless Searches and Arrests

Again, warrants usually aren’t needed for arrests. They’re required more often for searches. But even when it comes to searches, there are plenty of situations where the officer doesn’t need a warrant. For instance, law enforcement typically need not have a search warrant:

• to search someone immediately after arrest
• where there’s some kind of emergency or “exigency” (as where the suspect is about to destroy evidence)
• when the person to be searched consents, or
• for a “stop and frisk.”

Laws Differ

Search and seizure law tends to be pretty consistent throughout the United States. All law enforcement agencies, federal and state, have to abide by the Fourth Amendment. But there can be slight differences between jurisdictions—meaning, for instance, that evidence that’s admissible in court in one state might not be in another. Make sure to consult an experienced criminal defense attorney to understand how the law applies to your situation.