The Commission on Macroeconomics and Health recommended a significant expansion in funding for health interventions in poor countries. However, there are a range of constraints to expanding access to health services: as well as an absolute lack of resources, access to health interventions is hindered by problems of demand, weak service delivery systems, policies at the health and cross-sectoral levels, and constraints related to governance, corruption and geography. This special issue is devoted to analysis of the nature and intensity of these constraints, and how they can best be overcome