This dissertation examines the formation and evolution of the nuclear security regime (NSR) which governs programs, policies and norms associated with the protection of nuclear and radiological materials that sub-state groups might seek to acquire for use in a terrorist attack. The regime is unique in the security field in that it is an ad hoc, voluntary structure that more closely resembles regimes associated with environment protection than the institutionalized, internationally negotiated treaties and alliances that are typical of regimes governing national security affairs. The dissertation reviews how the NSR developed over 16 years and spanned U.S presidential administrations with generally opposing approaches to multilateral cooperation and global governance. It is divided into five chapters that describe the regime; places its development within the academic context of regime theory and multilateral cooperation; traces the regime's evolution from nonproliferation efforts to counterterrorism ones; looks at the specific approaches enacted by the George W. Bush and Barack H. Obama administrations; and concludes with observations on the regime's continued longevity.