This thesis examines Romania's Cold War acquisition of nuclear technology from Western suppliers during the period of détente. This first account of the Romanian nuclear programme is based on multi-archival research that relies extensively upon recently declassified documents from both sides of the former Iron Curtain. It complements supplier-centric explanations of nuclear assistance and restores agency to recipient countries, in this case Romania. The thesis argues that Romania was an 'international nuclear entrepreneur' which turned to the West for atomic assistance because Moscow delayed transfers of the technology that the leadership in Bucharest had requested. Romania then discovered that the Soviet policy of 'denial by postponement' contrasted with more positive attitudes in the West. Over this period, Romania managed to secure nuclear technology thanks to its ingenuity and its political entrepreneurship with the West. In particular, Romania obtained atomic assistance by offering its good offices to the United States during the Vietnam War and the Sino-American rapprochement, and by playing West European nuclear suppliers off against one another. This thesis thus offers a new interpretation of how and why a country like Romania could gain the access it did to Western nuclear technology during these years, revealing lessons about nuclear acquisition strategy and non-proliferation more generally. It challenges the conventional view of atomic assistance, which holds that superpower nuclear suppliers provided nuclear assistance because of an ideology shared with recipient countries and because they felt pressured by international institutions. This historical analysis is based upon a new, multi-archival reading of primary sources, and contributes to a post-revisionist and more nuanced interpretation of the 'nuclear' dimension of the détente period in the Cold War by examining the remarkable agency of one smaller East European power.