The debate over nuclear proliferation has generated a large literature, but this literature does not adequately address the moral issues. A moral analysis of proliferation must go beyond concerns of international security. In this essay, Lee addresses the following questions: (1) Does nuclear proliferation make the world a more dangerous place; that is, does it increase security? (2) Is it morally permissible for a nonnuclear state to acquire nuclear weapons? (3) What are morally permissible actions for states trying to keep other states from acquiring nuclear weapons?