Private sector providers are the most commonly consulted source of care for child illnesses in many countries, offering significant opportunities to expand the reach of essential child health services and products. Yet collaboration with private providers presents major challenges - the suitability and quality of the services they provide is often questionable and governments' capacity to regulate them is limited. This article assesses the actual and potential contributions of the private sector to child health, and classifies and evaluates public sector strategies to promote and rationalize the contributions of private sector actors. Governments and international organizations can use a variety of strategies to collaborate with and influence private sector actors to improve child health - including contracting, regulating, financing and social marketing, training, coordinating and informing the public. These mutually reinforcing strategies can both improve the quality of services currently delivered in the private sector, and expand and rationalize the coverage of these services. One lesson from this review is that the private sector is very heterogeneous. At the country level, feasible strategies depend on the potential of the different components of the private sector and the capacity of governments and their partners for collaboration. To date, experience with private sector strategies offers considerable promise for children's health, but also raises many questions about the feasibility and impact of these strategies. Where possible, future interventions should be designed as experiments, with careful assessment of the intervention design and the environment in which they are implemented.