The private sector is an important supplier of public health products (PHPs) in developing countries. Although there are concerns about the quality and affordability of these products, private providers also offer possibilities for expanding access to key commodities. This paper proposes a conceptual framework for understanding the public health implications of private sales of PHPs. It reviews methods for studying these sales, together with their advantages and shortcomings. Ten methods are identified which can be used for studying the behaviour of providers and consumers. The effects of seasonal variation are discussed, together with the challenges of creating a sampling frame and studying illicit behaviour. We conclude that relatively little is known about the sales of PHPs, that more is known about contraceptives and drugs than about the newer products, and that the demand side of the market has been studied in greater depth than the behaviour of suppliers. The existing toolbox is biased towards formal providers, and thus, probably towards understanding the provision of PHPs to those who are better off. Methods for studying the supply of PHPs in outlets used by poor people is a priority area for further methodological development.