This dissertation project examines the Social Protection Floor Initiative (SPF-I), a global social policy initiative that brought together various international organizations which have traditionally had divergent social policy approaches. Since the launch of the SPF-I in 2009, most of the major international organizations in the development field became part of the Initiative and engaged in the Social Protection Floor (SPF) policy at varying levels. This project focuses on six international organizations, namely the United Nations Children’s Fund (UNICEF), the United Nations Development Programme (UNDP), the World Health Organization (WHO), the Organization for Economic Cooperation and Development (OECD), the World Bank, and the International Monetary Fund (IMF). The overarching research question is, “what explains the extent of, and the variation in, the international organizations’ adoption of a given policy (in this case, the SPF)?” In order to explain the extent of policy adoption in international organizations, a policy adoption matrix has been developed. This matrix helps to identify each international organization as a policy leader, policy follower, or policy supporter based on the following parameters: the speed and the timing of policy adoption, the level of commitment, the breadth of organizational buy-in, and the scope of policy adoption. While the UNICEF, the UNDP, and the WHO are identified as policy leaders, the OECD is identified as a policy follower, and the World Bank and the IMF are identified as policy supporters. Next, this study explains why these international organizations are policy leaders, followers, or supporters. The analysis of policy adoption in the six international organizations reveals that: (i) the presence of policy entrepreneurs within relevant networks and the policy’s good fit in the organizations’ outlook explain why the UNICEF, the UNDP, and the WHO are policy leaders, (ii) the role of member states and the policy’s poor fit in the organization’s outlook explain the OECD’s role as a policy follower, and (iii) the external pressures and the policy’s poor fit in the organizations’ outlook explain why the World Bank and the IMF are policy supporters. This analysis concludes with an analytical framework towards a theory of policy adoption in international organizations.