An American epistemic community played a key role in creating the international shared understanding and practice of nuclear arms control. In the absence of nuclear war, leaders' expectations of nuclear war and of its control were affected by causal theories and abstract propositions and models which, given their “scientific” and technical nature, were developed by an epistemic community. This study, which emphasizes the roles played by epistemic communities in policy innovation and in the diffusion of understandings across nations and communities, analyzes how the theoretical and practical ideas of the arms control epistemic community became political expectations, were diffused to the Soviet Union, and were ultimately embodied in the 1972 antiballistic missile (ABM) arms control treaty. In contrast to those studies that have concentrated primarily on the workings of international epistemic communities, this study stresses the notion that domestically developed theoretical expectations, which were worked out by a national group of experts and selected by the American government as the basis for negotiations with the Soviets, became the seed of the ABM regime. Moreover, by suggesting that the arms control epistemic community was really an aggregation of several factions that shared common ground against various intellectual and policy rivals, this study sheds light on the question of how much coherence an epistemic community requires. The political selection of new conceptual understandings, followed by their retention and diffusion at national and international levels, suggests an evolutionary approach at odds with explanations of international change advanced by structural realism and approaches based on it.