The Exclusionary Rule prevents the government from using most evidence gathered in violation of the United States Constitution. It applies to evidence gained from an unreasonable search or seizure in violation of the Fourth Amendment, see Mapp v. Ohio, 367 U.S. 643 (1961), to improperly elicited self-incriminatory statements gathered in violation of the Fifth Amendment, see Miranda v. Arizona, 384 U.S. 439 (1966), and to evidence gained in situations where the government violated defendants’ Sixth Amendment Right to Counsel, see Miranda. The rule does not apply to civil cases, including deportation hearings. See INS v. Lopez-Mendoza, 468 U.S. 1032.
If evidence that falls within the scope of the exclusionary rule led law enforcement to other evidence, which they would not otherwise have located, then the exclusionary rule applies to the related evidence found subsequent to the excluded evidence as well. Such subsequent evidence has taken on the name of “fruit of the poisonous tree.”
The Exclusionary Rule is a court-created remedy and deterrent, not an independent constitutional right. Courts will not apply the rule to exclude illegally gathered evidence where the costs of exclusion outweigh its deterrent or remedial benefits. Thus, the rule is not triggered when courthouse errors lead police officers to mistakenly believe that they have a valid search warrant, because excluding the evidence would not deter police officers from violating the law in the future. See Arizona v. Evans, 541 U.S. 1.
The rule does not prevent the government from introducing illegally gathered evidence to “impeach,” or attack the credibility of, defendants’ testimony at trial. The Supreme Court recognized this exception to prevent perjury. Even when the government suspects perjury, however, it may only use tainted evidence for impeachment, and may not use it to show guilt.
Due to qualified immunity, the Exclusionary Rule is often defendants’ only remedy when police officers conduct an unreasonable search or violate their Miranda rights. Even if officers violate a defendant's constitutional or statutory rights, qualified immunity protects them from suit unless no reasonable officer would believe that their conduct was legal.