This book offers a novel approach to understanding the puzzle of nuclear proliferation by examining how leaders’ beliefs and perceptions about the international system influence states' decisions to acquire nuclear weapons. Today, there is a persisting dilemma over the spread of nuclear weapons for both practitioners and scholars of international affairs. Uncertainty remains whether determined proliferators can be stopped, as shown by the cases of North Korea and Iran. These instances of proliferation raise questions about regional stability, the use of pre-emptive military action, and the potential for reactive-proliferation by neighbouring countries. Despite the serious implications surrounding the spread of these weapons, proliferation scholarship has thus far failed to solve what has been described as the "proliferation puzzle"- why do some countries choose nuclear weapons while others do not? The author argues that understanding basic psychological motivations, such as the role of power and perceptions of self and others, forms a strategic context which provides answers about a leader’s willingness to proliferate. Proliferation willingness is a critical, yet frequently overlooked, part of the proliferation equation. Ultimately, it is the combination of willingness and proliferation opportunity (i.e. technical and scientific capabilities) that determines whether a country 'goes nuclear'. By examining several historical instances of proliferation decision-making—in South Africa, India, Libya and Australia—the book's findings highlight the fundamental role of leaders’ beliefs in shaping proliferation outcomes. This book will be of much interest to students of nuclear proliferation, political psychology, security studies and IR in general.