This chapter traces the history and development of the concept of “reasonable expectations of privacy” over three stages: its conception in the 1967 U.S. Supreme Court judgment Katz v. United States, its further development in subsequent Fourth Amendment jurisprudence, and finally its migration and expansion to other jurisdictions around the globe. This historical analysis not only reveals why the concept has been adopted across so many legal systems. It also provides a first impression of the notion’s different possible uses and implications and pinpoints some initial critiques of the concept (as brought forward especially in the American context).