Katz v. United States
Katz v. United States, 389 U.S. 347 (1967) is the leading precedent requiring a warrant before the government can place a wiretap (recording device) on a phone line, even a phone line connected to a public telephone booth.
- [F]irst that a person have exhibited an actual (subjective) expectation of privacy and, second, that the expectation be one that society is prepared to recognize as "reasonable." Thus a man's home is, for most purposes, a place where he expects privacy, but objects, activities, or statements that he exposes to the "plain view" of outsiders are not "protected" because no intention to keep them to himself has been exhibited. On the other hand, conversations in the open would not be protected against being overheard, for the expectation of privacy under the circumstances would be unreasonable.
- 389 U.S. at 361 (Harlan, J., concurring)