Courts need a way to enforce the Fourth Amendment, which prohibits unreasonable searches and seizures. The principal method they use is the exclusionary rule, which provides that evidence officers obtain in violation of the Fourth is generally inadmissible in a subsequent prosecution. (To read more about this concept, see Fruit of the Poisonous Tree.)
But what is a judge to do when the actions were improper, but the motive wasn't? For example, what happens when a police officer finds cocaine on someone he reasonably—but mistakenly—believes to be subject to arrest and search?
The U.S. Supreme Court introduced the good faith exception for situations like these, where there appears to be a lawful basis for a search, but isn't. This exception often comes into play when officers reasonably rely on search warrants that turn out to be invalid.
(For information on the effect of an officer's misunderstanding of the law, see Police Detentions Based on Mistakes of Fact and Mistakes of Law.)
The good faith exception ostensibly requires that all members of law enforcement who are involved in a search behave properly. If, for instance, one officer fudges the facts in an application for a search warrant that would've otherwise been insufficient and another conducts a search in reasonable reliance on it, the good faith exception doesn't apply. Likewise, if the warrant, on its face, is vague enough to lead a reasonable police officer to believe it's invalid, evidence found pursuant to it will be inadmissible.
But courts tend to rule in favor of good faith. The exception usually applies, for instance, where police employees have made errors in maintenance of warrant databases, leading officers to search the wrong people. Courts also invoke good faith when officers rely on law that later changes. For example, if officers attach a GPS to a car without a warrant because existing law allows them to, but a later Supreme Court decision holds that warrants are required, evidence found pursuant to the GPS search will probably be admitted. (U.S. v. Sparks, 711 F.3d 58, 68 (1st Cir. 2013).)
Courts reason that the purpose of the exclusionary rule—to deter officers from misbehaving—isn't served by keeping out evidence when cops have behaved honestly and reasonably. Of course, critics assert that no matter the reason for a law enforcement mistake, citizens shouldn't bear the brunt of it. ("Aren't we otherwise sanctioning law enforcement incompetence?") Further, they suspect that officers carry out searches under the auspices of good faith, but with the true intention of circumventing the Constitution.
Some states, interpreting their own laws rather than the federal Constitution, choose not to apply the good faith exception. Some resort to it only under limited circumstances, while others flatly reject it. (The U.S. Constitution sets a minimum standard for protecting individual liberty—states are free to provide more protection pursuant to their own constitutions.)
If you face charges related to evidence officers obtained through a search (with or without a warrant), ask your attorney about the exclusionary rule. A knowledgeable criminal defense attorney will be able to explain the applicable law in your jurisdiction and analyze any potential attempts by the prosecution to invoke good faith or another search exception.