Chapter 4 examines U.S. nonproliferation policy beginning in the Cold War, as well as nonproliferation policy after the collapse of the USSR in the “second nuclear age,” an era characterized by a range of state and nonstate actors seeking to acquire a WMD capability and expansion of U.S. policy to include unilateral and bilateral tools to plug the gaps in the multilateral nonproliferation regime. Key terms include: proliferation, Nuclear Nonproliferation Treaty, sanctions, P5+1, highly enriched uranium.