Katz v. United States (1967)
With its decision in this case the Supreme Court reversed Olmstead v. U.S. and brought electronic surveillance within the Fourth Amendment's prohibition of unreasonable searches and seizures.  The case involved a listening device installed in a public telephone booth.  Government agents listened in only when the suspect was using the phone.  The court ruled that a conversation was a "thing" which could be seized and that the government agents needed to get a warrant before installing the wiretap.

http://www.oyez.org/cases/1960-1969/1967/1967_35/