The thesis explores the nature of executive leadership in international organisation. Executive leadership is often praised or blamed for outcomes in international agencies, and yet, the disciplinary literature fails to incorporate the executive head into institutional analyses of politics, power, and change over time. The thesis aims to address this lacuna and the role of executive leadership by analysing if and how it matters in international politics. The thesis draws on a composite literature from other areas of political research to establish what is known. A review of the literature and prevailing approaches to leadership studies reveals that an overwhelming majority of scholarship relies on exclusively structural or agential accounts of leadership. This somewhat determinist literature has distorted the limited knowledge on the nature of executive leadership in international organisation. Approaches that focus on agency-based explanations argue that executive heads matter greatly. Approaches that utilise structure to interpret executive leadership find that it matters little, if at all. Rejecting these narrow frameworks, the thesis uses a dialectical approach, supported by critical realism, to analyse four cases of executive leadership in the World Trade Organization to address the research questions and lacuna. The case studies draw on over 70 years of multilateral trade governance to reveal a set of core and subsidiary findings about politics, power, executive leadership, and change over time. The thesis argues that executive leadership matters, but that how it matters is contingent on the executive head and the circumstances of their term. By incorporating the executive head into the disciplinary literature, the thesis argues politics, power, and change over time can be more accurately understood.