The adoption of UN Security Council Resolution (UNSCR) 1540 in 2004 reflects an emerging consensus that more should be done by the international community to address the proliferation of weapons of mass destruction. UNSCR 1540 articulates a universal, legally binding obligation for all states to confront proliferation by adopting effective export control systems. To date, however, there have been no attempts to systematically analyze compliance with this new obligation, making it impossible to assess the success of this measure and the underlying causes of any shortcomings. This study addresses this by conducting a systematic empirical analysis of state compliance with UNSCR 1540. Drawing upon theories of compliance with international law, we investigate two distinct explanations for variation in the degree to which states adopt nonproliferation export controls: one based on state interests and enforcement and the other based on state capacity. Our statistical tests of these theories use a new, cross-national data set detailing the nonproliferation policies of 30 states. The empirical results indicate that compliance with international nonproliferation obligations is influenced most by a state's economic and governmental capacities and has little to do with interest-based factors. These findings suggest that capacity-building programs are the best option for improving the implementation of UNSCR 1540 and of nonproliferation efforts in general.