The policy making approaches, social policy agendas and instruments deployed by large, Western philanthropic donors - referred to as big philanthropies in this study - have tangible consequences for the institutions, mechanisms and processes of social policy formation and social policy delivery across the Global South. Over the past decade, big philanthropies have also become increasingly integrated and influential actors in the processes and structures of global social governance. Given their significance in shaping global social policy trends as well as aid architecture and welfare outcomes in the Global South, this research seeks to contribute to a better understanding of the social policy approaches deployed and promoted by contemporary philanthropic donors, as well as the theoretical frameworks that explain them. This doctoral thesis examines these issues through a qualitative case study of the Bill and Melinda Gates Foundation's work in the health sector in Tanzania. Two primary research interests guide the analysis: (i) understanding whether the ideologies, policy goals and instruments promoted by big philanthropies represent a coherent and distinctive global social policy paradigm; and, (ii) identifying the sources of authority and strategies deployed by the Gates Foundation to instigate transnational social policy transfer. The research also explores the perspectives of a wide range of stakeholders on the comparative advantages and potential risks of social policy delivery by big philanthropies in a development context. Crucially, in addition to interviews with representatives of the Gates Foundation itself, the study examines the views and experiences of the government and civil society actors on the ground in Tanzania. The analysis is primarily guided by: Peter Hall's theory of policy paradigms; policy transfer analytical frameworks; and Global Social Policy theory. The findings of the thesis suggest that big philanthropies are significant policy actors and operate within a coherent social policy model. Underpinned by the view that poor populations lack the necessary assets to fully realise their economic capacity, the model constitutes an instrumental 'social investment' approach. It is primarily focused on strengthening individual capacity in order to spur employment, productivity and economic growth, in contrast to approaches that pursue social justice or social equality through universal welfare systems. It is also differentiated from other paradigms by its emphasis on private sector methods and targeted vertical interventions (focusing on specific diseases or targets as opposed to broader system-wide objectives) that prioritise innovation, use of data and technology. The overall method is analogous to Third Way models - driven by short-term pragmatism rather than dogma, and promoting social policy approaches that integrate private, public and third sector actors. The analysis also shows that big philanthropies have unique forms of normative and epistemic authority, derived from the legitimacy of altruism, celebrity status, up-to-date scientific knowledge and private sector expertise. Combined with extensive access to different cross-sectoral networks, these characteristics enhance their policy influence as global policy entrepreneurs. The evidence also demonstrates that the Gates Foundation's policy transfer strategies combine high visibility policy promotion, covert influence notably through local policy intermediaries, and subtle financial pressure through 'soft conditionalities'. Local stakeholder perspectives pointed to several risks and caveats associated with the involvement of big philanthropies, and the Gates Foundation specifically. These included the omission of national policy plans and priorities, exacerbation of fragmentation of development efforts, and friction with the domestic policy environment - potentially leading to inappropriate policy transfer. This thesis underscores the fact that external actors - including Western big philanthropies - continue to influence social policy directions in the Global South and that this renders their involvement problematic. The study also demonstrates the robustness of policy transfer frameworks in explaining this phenomenon and shows that social policy ideas and approaches travel transnationally both through epistemic-hegemonic influences and hard mechanisms drawing on financial pressure.