My doctoral project investigates the impact of Ronald Reagan’s Strategic Defense Initiative on transatlantic relations during the period 1983-86. The dissertation focuses on the three main European powers, namely Britain, France, and the Federal Republic of Germany, and examines their reaction to SDI both individually and comparatively. The study exploits SDI’s position at the intersection of nuclear strategy, political ideology, Cold War diplomacy, and industrial politics to offer a multifaceted, multi-national, and primary source-based analysis of US-European relations during the Reagan Presidency. The picture of the transatlantic relationship which emerges from the dissertation is a complex and nuanced one. On the one hand, the analysis argues that relations across the Atlantic during the Reagan era cannot be reduced to a scenario of accelerating ‘drift’ between the United States and Western Europe. Instead, on SDI as well as on other matters, moments of acute friction alternated with a constantly renewed search for dialogue, cooperation, and compromise on the part of the Europeans and also, if to a lesser degree, of the Americans. On the other hand, the ‘exceptionalist’ ideology and worldview underpinning SDI, the prevailing indifference in Washington to its implications for NATO, and most importantly the persistent anti-nuclear rhetoric and ambitions associated with the initiative revealed a distinct lack of sensitivity to European interest by the Reagan administration. As the dissertation shows, the anti-nuclear drive inherent in SDI, which both reflected and reinforced Reagan’s deep-seated interest in nuclear abolition, constituted the most disruptive aspect of the initiative from the viewpoint of European leaders. In these respects, the SDI controversy epitomises the unilateral tendencies and increasingly divergent priorities from those of the European allies which characterised much of the Reagan administration’s foreign policy – making the 1980s a decade of recurrent tensions in transatlantic relations.