Katz v. United States

The government needs a warrant to listen to your private communications even in a public place if you have a reasonable expectation of privacy. This established the "reasonable expectation of privacy" test used in all search-and-seizure cases.

The Fourth Amendment protects people, not just places. Warrantless wiretapping of a public phone booth violated the Fourth Amendment.

Foundation for modern privacy law. Established that the Fourth Amendment protects private communications, not just physical spaces.