This article examines the Apollo-Soyuz Test Project in 1975 as an instrument of diplomacy and as a catalyst for East-West détente. The topic has received little attention in either the general literature on the Cold War—which has only recently begun to address the political significance of science and technology more generally—or in the literature on space history, which has focused mostly on the earlier race to land on the moon and has devoted little attention to the collaboration in space that has dominated crewed space missions from the 1970s, leading up to the International Space Station. The article connects two previously separate spheres of study—space history and diplomatic history—to shed light on the importance of space exploration in the bigger story of Cold War diplomacy.