Human resources are the crucial core of a health system, but they have been a neglected component of health-system development. The demands on health systems have escalated in low income countries, in the form of the Millennium Development Goals and new targets for more access to HIV/AIDS treatment. Human resources are in very short supply in health systems in low and middle income countries compared with high income countries or with the skill requirements of a minimum package of health interventions. Equally serious concerns exist about the quality and productivity of the health workforce in low income countries. Among available strategies to address the problems, expansion of the numbers of doctors and nurses through training is highly constrained. This is a difficult issue involving the interplay of multiple factors and forces.