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In addition to general evidence rules, criminal defendants benefit from the protection of the exclusionary rule, which makes all evidence obtained through unlawful police investigation inadmissible. The Fourth Amendment of the US Constitution forbids the federal government, state governments*, and any of their agents from conducting unreasonable searches or seizures. Searches and seizures generally must be supported by a warrant issued by a court, but there are exceptions. Below, this article first addresses the role that the exclusionary rule plays, and second, what constitutes an unreasonable search and seizure.
Let us first clarify how these rules play into a criminal proceeding. Courts long ago adopted the exclusionary rule largely on the theory that the rule would motivate police to conduct searches lawfully and require the state to respect individual right. The rule does not only apply to evidence first seized by unlawful means, but also applies to any found that would not have otherwise have been found without the unlawfully attained evidence.
That's the theory. What about practice?
If there are questions as to whether evidence is to be excluded, especially if the prosecution would otherwise have no evidence, a criminal defendant's lawyer will likely request a motion in limine. Such a motion's effect is to request an evidentiary hearing between the indictment (where the defendant is formally accused) and the trial. At this hearing, the defendant has an opportunity to prove that evidence will be inadmissible at trial. At the hearing, witnesses will be called, evidence will presented, arguments will be made, and the judge will decide whether the evidence is admissible. This has the beneficial effect of keeping inadmissible evidence that the prosecution could attempt to admit at trial, tainting the jury, and also allows the attorneys to prepare their case knowing which pieces of evidence will be admissible. Of course, it is always permissible to object to evidence using the exclusionary rule at trial, but the defendant runs the risk of a tainted jury and not being granted a new trial.
Keep in mind that most criminal cases end in a plea bargain, but the prosecution's bargaining position will be weakened if there is a strong chance key evidence will be inadmissible. An unreasonable search and seizure generally is any search not supported by a warrant issued upon probable cause (i.e. probable cause the search will uncover criminal activity or contraband). Police officers are expected to get a warrant if circumstances allow, but there are exceptions that in theory are to be read narrowly. The exceptions and the general feel of the law in this area are governed by the overarching idea of expectations of privacy. Individuals have no expectation of privacy of items laying in their front yard by the sidewalk, have lower expectations of privacy in their car, and have very high expectations of privacy in their home. Accordingly, a police officer can seize anything that he or she believes to be contraband that is in plain view from a public area (the plain-view rule); vehicles can be stopped upon reasonable suspicion (of criminal activity); and probable cause is required to search inside the home.
More concretely, here are some commonly applicable rules:
Vehicles
If a vehicle is stopped without any reasonable suspicion, any evidence obtained during the stop will be inadmissible, barring misbehavior of the defendant. In a lawful stop, an officer can seize items that are plain sight, but needs probable cause to search hidden areas like the trunk or glove box.
Protective sweep
If a police officer believes that there is imminent danger to his or her own life or the lives of others, they can enter an area that he or she would otherwise need a warrant to enter. During this "sweep," to the extent the sweep is confined to areas that there is reasonable necessity to search, the evidence found will be admissible.
Stop and frisk
If a police officer believes there is criminal activity afoot, that the person to be frisked is armed, and the person is posing danger to the officer or others, items found in a frisk reasonably limited to searching for weapons will be admissible.
Plain-view
If a police officer can see contraband from a place that the police can otherwise lawfully be, the police officer can seize the contraband.
Consent
If a person authorized to consent to a search does so, the results of the search will be admissible. A person can only consent to a search of their own property, so authorization will be a question, if someone other than the defendant consents.
Searches incident to lawful arrest
Police officers may lawfully search the area within immediate control of the arrestee to prevent the arrestee from obtaining weapons and destroying evidence.
These rules are meant as just a sampling of the applicable rules on unreasonable searches and seizures and only skim the surface of the applicable law. *The Fourth Amendment applies to the states through the Fourteenth Amendment.