U.S.-Russian cooperation on nuclear materials, protection, control, and accounting (MPC&A) grew rapidly in the 1990s, but then stagnated. What explains this patterm, and what steps could be taken to revitalize joint efforts to secure nuclear material? This article contends that MPC&A cooperation is most effective when government officials set overall policy goals, lobby for political support and funding, and provide central coordination, while scientists build trust with their Russian counterparts, develop technical plans, and oversee implementation on the ground. MPC&A cooperation has faltered since the late 1990s primarily because this ba;ance has shifted too far in favor of bureaucratic control in both the United States and Russia. The article leadership, and offers lessons from U.S.-Russian MPC&A cooperation for possible future dealings with Iran, North Korea, and othwr proliferant states.