This thesis investigates Japan's contribution to United Nations (UN)-sponsored peacekeeping operations (UNPKO) by locating sources of activism and passivism in Japan's foreign policymaking process. In particular, it examines the influence of factors, such as Japan's traditional post-W.W.II commitment to pacifism, its relationships with the US and its East Asian neighbours, and the role of the UN. The introduction provides a broad overview of the remit of the thesis as well as clarifying its ontological commitments and justifying the topics of focus, Japan and the UN. Chapter One constructs a detailed theoretical approach to this topic by rejecting traditional realist, liberal, and Marxist interpretations of international politics and, instead, highlighting the study of norms in international society. Chapter Two centres on the topic of UN peacekeeping operations and explains how this practice has become a norm of international society. Chapter Three introduces the topic of Japan's foreign policy by examining traditional approaches and interpretations. It also utilises the approach outlined in Chapter One and examines Japan's contribution to PKO from the time of admission to the UN in 1956 through to the eve of the outbreak of the Second Gulf War. Chapter Four looks at Japan's response to the Second Gulf War from the financial contribution through to the legislation adopted to facilitate the despatch of the Self-Defence Forces (SDF). It demonstrates the initial power of traditional norms in shaping policy and how this changed with the rise of the influence of the UN. Chapter Five takes the first despatch of the SDF to Cambodia as its case study and reveals how the traditional norms of domestic-rooted pacifism and the opposition of East Asian nations to Japanese re-militarisation continued to be eroded. Chapter Six looks at the most recent of the SDF's despatches to Mozambique, Rwanda and the Golan Heights and demonstrates the continued influence of the US as well as the consolidated power of the UN, in contrast to the declining influence of pacifism and Japan's East Asian neighbours. Taking this empirical investigation into account, the conclusion reappraises the importance of norms in Japan's foreign policy making process, and highlights the influence of the UN.