In low and middle income countries, health workers are essential for the delivery of health interventions. However, inadequate health-worker performance is a very widespread problem. We present an overview of issues and evidence about the determinants of performance and strategies for improving it. Health-worker practices are complex behaviours that have many potential influences. Reviews of intervention studies in low and middle income countries suggest that the simple dissemination of written guidelines is often ineffective, that supervision and audit with feedback is generally effective, and that multifaceted interventions might be more effective than single interventions. Few interventions have been evaluated with rigorous cost-effectiveness trials, and such studies are urgently needed to guide policy. We propose an international collaborative research agenda to generate knowledge about the true determinants of performance and about the effectiveness of strategies to improve performance. Furthermore, we recommend that ministries of health and international organisations should actively help translate research findings into action to improve health-worker performance, and thereby improve health.