Complex humanitarian emergencies have been a major political, security and public health feature of the post-Cold War world. These man-made disasters account for more morbidity and mortality than all natural and technological disasters combined. In order to deliver effective aid during complex humanitarian emergencies, international relief agencies must have a solid understanding of the political and social climates in which they are operating. In addition, they should base their health interventions on objective epidemiological data, especially standardized rates of morbidity and mortality. Most deaths during complex humanitarian emergencies are due to preventable causes, especially increased rates of infectious diseases malnutrition and violent trauma. The most appropriate health interventions are therefore based on the models of public health and primary health care, emphasizing disease prevention and health promotion. The field of humanitarian assistance has become increasingly professionalized in recent years, with its own professional standards, literature, research agenda and training opportunities. It is an unfortunate reflection on the current state of international affairs that the number of complex humanitarian emergencies and the enormous levels of suffering associated with them are unlikely to decline in the foreseeable future. See Commentary, page 143.