For all its notoriety and controversial character, the history of Swiss banking secrecy remains largely unexplored. This article traces the crucial phases of its development. It reveals that the maintenance and reinforcement of banking secrecy represented a major objective of Swiss authorities throughout the twentieth century, and exerted a substantial influence on Swiss domestic and foreign policy. It demonstrates that, contrary to popular opinion, the institution of Swiss banking secrecy did not arise from a desire to protect the funds deposited in Switzerland by Jewish victims of Nazi persecution but rather had substantially different origins. Moreover, this article shows that Swiss banking secrecy gave rise to tensions between Switzerland and the Great Powers, especially during and after World War II, when it poisoned relations between Switzerland and the United States.