This chapter focuses on how Barack Obama sought to reduce the role of nuclear weapons in US national security, makes changes to nuclear doctrine, and engages with the global disarmament movement in order to review the various successes and failures of the Obama administration's nuclear agenda, and to consider its legacy. It evaluates Obama's attempts to reignite the nuclear arms reduction process with Russia, notably by the 2010 New Strategic Arms Reduction Treaty (START). The chapter examines how the Obama administration has sought to address nuclear proliferation challenges - namely North Korea and Iran - and its wider re-engagement with the Nonproliferation Treaty (NPT) regime. It then explains the new measures designed to combat the threat of nuclear terrorism and to enhance global nuclear security. In the realm of nuclear security, Obama embraced several of the precepts of Bush's nuclear security strategy, notably the Proliferation Security Initiative (PSI) and the Global Initiative to Combat Nuclear Terrorism (GICNT).