States that have acquired nuclear weapons must confront the complicated and important question of how to structure their nuclear arsenals. 2 Some states, such as the United Kingdom, field only a small number of nuclear platforms, while others, such as the United States and the Soviet Union, establish diverse portfolios of weapons with varying range, destructive power, and other characteristics. 3 Nuclear states differ dramatically not only in the number of nuclear platforms they deploy, but also in the relative weight they place on particular weapons systems and on each component of the nuclear triad (air, land, and sea-based weapons). 4 These characteristics have also changed over time—nuclear forces that seem appropriate in one strategic environment may be made redundant or obsolete by the introduction of new technologies or by cycles of crisis and détente. Variation across nations and time raises several key questions: Why do states deploy the nuclear force structures they do? What drives the decisions of states to invest in new nuclear platforms? How do officials think about the diversification of their nuclear portfolios?