East Asia and East Asian regionalism have gained greater attention. Given this, what makes this region and who determines its shape are very important questions, which are, in turn, highly relevant to questions of regional leadership. This thesis thus aims to examine and explain the nature of Sino-Japanese regional leadership and explore its impacts on the shape of the East Asia region and East Asian regionalism. It does so particularly with reference to the CMI and regional organisation-building from the APT to the EAS. The thesis explores two key themes. First, it seeks to bridge a gap in the study of East Asian regionalism in particular and East Asia in general by focusing on the dynamics of Sino-Japanese leadership competition. There is still a lack of a dedicated study to examine the dynamics of regional leadership in the region and its impact on the East Asia region and East Asian regionalism. It addresses why regional powers assert regional leadership and how their assertions of regional leadership change their interests and behaviours with regard to regional cooperation. By doing so, it can help better comprehend the interests and strategies of China and Japan and their impacts on the shape of East Asian regionalism. Secondly, this thesis aims to fill in a gap in the study of global or regional leadership by developing a constructivist analytical tool to define leadership and examine the dynamics of leadership. It highlights that neither the realist and liberal approaches to international leadership nor the emerging literature on regional powers provides a good analytical tool to conceptualise regional leadership and to examine the dynamics of regional leadership competition. It argues that some insights of constructivism help to better comprehend the dynamics of regional leadership.