The proliferation of public-private partnerships is rapidly reconfiguring the international health landscape. This article (part I of two on the subject) traces the changing nature of partnership, and discusses the definitional and conceptual ambiguities surrounding the term. After defining global public-private partnerships (GPPPs) for health development, we analyse the factors which have led to the convergence of public and private actors and discuss the consequences of the trend toward partnership between UN agencies (including the World Bank) and commercial entities in the health sector. Generic factors such as globalization and disillusionment with the UN, and factors specific to the health sector, such as market failure in product development for orphan diseases, are examined. Reviewed are the interests, policies, practices and concerns of the UN, the private-for-profit sector, bilateral organizations, and governments of low-income countries with respect to public-private partnership. While GPPPs bring much needed resources to problems of international health, we highlight concerns regarding this new organizational format. Part II, which will be published in the May issue of the Bulletin, presents a conceptual framework for analysing health GPPPs and explores the issues raised.