Illinois v. Gates established the totality-of-the-circumstances test for evaluating probable cause.
In 1983, the Supreme Court held that magistrates should assess an informant’s tip and all corroborating evidence together, rather than applying a rigid checklist. The case involved an anonymous letter predicting the Gateses’ travel and alleged drug activity; police corroborated several details before obtaining a search warrant.
The ruling abandoned the separate Aguilar–Spinelli requirements that focused independently on an informant’s basis of knowledge and reliability. Those factors remain relevant, but a weakness in one may be offset by strong corroboration or other circumstances. The ultimate question is whether the facts establish a fair probability that evidence will be found.
This flexible approach is often confused with Terry v. Ohio, which concerns reasonable suspicion for brief investigative stops. Katz v. United States addressed privacy and searches, while Carroll v. United States concerned warrantless automobile searches. Gates primarily concerns probable-cause determinations, especially in warrant applications.