This article examines the evolution of middle power diplomacy on arms control during the Cold War. It argues that despite several attempts to influence major arms control negotiations, the structural constraints imposed by strict bipolarity—particularly during the early stages of the Cold War—limited the room for diplomatic manoeuvre by the small and medium-sized states. Factors such as the geographical voting groups within the United Nations system and the self-imposed discipline within traditional alliance structures typically restricted middle power initiatives on the important questions of international security such as arms control. Nonetheless, a number of efforts were made by leading middle powers such as Australia and Canada to progress the cause of arms control and significant policy ideas were at least canvassed during this time. The historical evidence shows that the middle powers were not innocent bystanders in the Cold War arms control debates, but whatever influence they had was ultimately s...