The Fourth Amendment to the US Constitution protects you against unreasonable searches and seizures. But how, exactly? Under what's commonly called the Fourth Amendment's "warrant requirement ," law enforcement agents need a warrant to search a person or place and seize evidence of a crime or illegal items. There are some exceptions to the warrant requirement, but, generally, a search conducted by law enforcement agents without a warrant, or in violation of a warrant, is unreasonable. And, under the exclusionary rule, any evidence that's obtained in violation of the Fourth Amendment - that is, through an illegal or unreasonable search - can't be used against you at trial.
Search Warrant Basics
A search warrant is a court order that authorizes law enforcement agents to search a particular place, like your home or office, and seize or take particular things, like evidence of a crime. Law enforcement agents have to ask a magistrate or judge for a search warrant. They do this by giving him a sworn statement, called an "affidavit," that explains why they think there's evidence of a crime in the place they want to search. If, based upon the affidavit, the magistrate or judge thinks that there's probable cause to search for and seize property, he'll give the agents a warrant. Generally, there's probable cause to search when all the facts and circumstances given in the affidavit convince the magistrate or judge that there's a fair probability that contraband or evidence of a crime will be found in the place that the agents want to search.
In addition, agents have to tell the magistrate or judge exactly where they want to search and what they want to seize. This is called the particularity requirement. Generally, once a warrant is issued, law enforcement agents can search only the area or place specified in the warrant and they can seize only the items specified in the warrant.
Exclusionary Rule in Action
The exclusionary rule is not in the US Constitution. Rather, the Supreme Court of the United States created the rule so that the Fourth Amendment's protection against unreasonable searches and seizures can be enforced. That is, the rule is designed to help make sure that law enforcement agents conduct searches and seizures properly.
How does it do this? If the police make an unlawful search and seizure and you're later charged with a crime, you can ask the court to exclude or "suppress" the evidence that was seized improperly. If the court agrees and finds that the police acted improperly, any evidence that they seized can't be used against you in court. The idea is simple: If law enforcement agents know beforehand that evidence obtained in violation of the Fourth Amendment can't be used against you in court, they'll work to make sure that they don't violate the Fourth Amendment.