The on-site inspection (OSI) regimes concluded in the last decade to verify major arms control agreements have established unprecedented levels of military cooperation and openness, added military predictability and stability, and provided new verification tools to deal with the proliferation priorities at the century's turn. This study is the first comprehensive effort to provide an overall understanding of how these regimes evolved, what they do, and how they contribute to national security goals. Rueckert concludes with a discussion of costs and benefits of the current approaches. A major professional tool for all those involved in arms control verification activities. Scholars and researchers in the fields of international relations and conflict resolution will find the work of interest as well.