This report is about the role of the private sector in mixed health systems. Mixed health systems have centrally planned government health services that operate side-by-side with private markets for similar or complementary products and services, which often existed long before the creation of national health ministries. The report emphasizes the importance of effective stewardship by governments of their country’s health system, especially given the reality that the private (nonstate) part of the system is large and complex, with major challenges and significant opportunities. It illustrates this case firstly through demonstrating that most developing countries’ health systems have active private actors in the sector. Secondly, it discusses possible government stewardship mechanisms for mixed health systems, describing mechanisms that developed country governments use to better manage and harness their health markets, while acknowledging the challenges of implementing such mechanisms in the developing world. The final section offers ideas for accelerating progress toward improved stewardship of mixed health systems.