Populations affected by armed conflict have experienced severe public health consequences mediated by population displacement, food scarcity, and the collapse of basic health services, giving rise to the term complex humanitarian emergencies. These public health effects have been most severe in underdeveloped countries in Africa, Asia, and Latin America. Refugees and internally displaced persons have experienced high mortality rates during the period immediately following their migration. In Africa, crude mortality rates have been as high as 80 times baseline rates. The most common causes of death have been diarrheal diseases, measles, acute respiratory infections, and malaria. High prevalences of acute malnutrition have contributed to high case fatality rates. In conflict-affected European countries, such as the former Yugoslavia, Georgia, Azerbaijan, and Chechnya, war-related injuries have been the most common cause of death among civilian populations; however, increased incidence of communicable diseases, neonatal health problems, and nutritional deficiencies (especially among the elderly) have been documented. The most effective measures to prevent mortality and morbidity in complex emergencies include protection from violence; the provision of adequate food rations, clean water and sanitation; diarrheal disease control; measles immunization; maternal and child health care, including the case management of common endemic communicable diseases; and selective feeding programs, when indicated.