The thesis poses two main questions: ‘what are NGOs?’ and ‘where do NGOs fit in?’. Taking the World Development Report of 1993 as the articulation of an international Health Sector Reform (HSR) agenda, the thesis questions the depiction of NGOs as private health service providers. Identifying four themes as key to the conceptualisation, design and implementation of HSR, it reflects on how these themes have been treated in health and development policy and practice. The thesis proposes that the ethos, programmes and actions of NGOs in health shed light on the interpretation of‘private*, ‘health’, ‘decentralisation’ and ‘community’ in ways that uncover important assumptions and contradictions inherent in the HSR agenda. It proposes that rather than taking NGOs to be private health service providers fitting neatly into increasingly market-based health systems, further exploration of the activist and advocacy role of NGOs is justified. Investigating HSR in Tanzania, the thesis shows that the dominant view of NGOs is as private service providers, but that, despite recognition of this role, NGOs have been largely excluded from policy discussions. The thesis takes Community Based Health Care (CBHC) - identified in the HSR policy documents and referred to by NGOs - as a case through which to explore the notion that NGOs are ‘community activists’, with a significance beyond direct health service provision. Promotion of CBHC by NGOs shows how they work in and around the formal health sector, at national, local government, and community levels. It concludes that whether providing primary health services, supporting selective primary health interventions, or promoting comprehensive empowerment-oriented approaches to primary health care, NGOs are embedded in complex processes of defining and meeting ‘public’ need. This merits the reconceptualisation of NGOs as public actors constantly defining, maintaining and developing their role through engagement in networks of public action