From the publisher: When and why do states transfer nuclear technology, materials, and knowledge to other states for peaceful purposes? This question is important given the recent finding that countries receiving nuclear aid are more likely to pursue and acquire nuclear weapons. I argue that countries provide civil nuclear assistance for three strategic reasons: to strengthen their allies and alliances, to strengthen their relationship with enemies of enemies, and to strengthen existing democracies and bilateral relationships with these countries (if the supplier is also a democracy). I test these arguments using statistical analysis and a new data set on more than 2,000 bilateral civilian nuclear cooperation agreements signed between 1950 and 2000. The findings offer robust empirical support for my argument and very little support for the competing explanation rooted in norms and nonproliferation. This article enhances scholarly understanding of how and why nuclear weapons spread and encourages further research on the supply side of nuclear proliferation. It also has broad implications for the literature on norms and international cooperation.