The authors explore the definition of SSR as it has emerged in the international community. The makeup of the security sector is examined, emergent principles are identified for implementing SSR in the community of practice, and the outcomes that SSR is designed to produce are specified. The supporting case studies of Haiti, Liberia, and Kosovo assess the impact of SSR programs on host nation security sectors. The authors conclude that those conducting SSR programs must understand and continually revisit the policy goals of SSR programs so as to develop concepts that support a transitional process that moves forward over time. Intermediate objectives required in support of this transition also articulate what is good enough and fair enough at various stages in the transformational process. State actors must acknowledge and often accommodate nonstate security actors more effectively in SSR planning and implementation, while recognizing both the advantages and the risks of collaborating with such actors. The authors also identify a need for rebalancing resources committed to SSR, especially given that justice and civil law enforcement typically are badly under-resourced as elements of SSR programs. Finally, the authors note the need for more flexible and better integrated funding processes to support SSR activities within the U.S. Government.