Why do countries deploy nuclear weapons abroad? Since 1945, more than 20 states have hosted foreign nuclear weapons on their territory, and five countries continue to do so today. These deployments have important consequences for international security, yet there is little systematic research about the factors that drive them. In this article, we develop three broad theoretical frameworks to explain why foreign nuclear deployments occur. Using a new dataset of foreign nuclear deployments between 1945 and 2000, we find that two factors weigh heavily in driving these deployments: the protection of allies and the projection of military power. Nonproliferation motives, however, appear to play little role. The results carry important implications for our understanding of the causes of proliferation and the sources of nuclear posture.