This article describes the technical basis for the guidelines for the integrated management of childhood illness (IMCI), which are presented in the WHO/UNICEF training course on IMCI for outpatient health workers at first-level health facilities in developing countries. These guidelines include the most important case management and preventive interventions against the leading causes of childhood mortality--pneumonia, diarrhoea, malaria, measles and malnutrition. The training course enables health workers who use the guidelines to make correct decisions in the management of sick children. The guidelines have been refined through research studies and field-testing in the Gambia, Ethiopia, Kenya, and United Republic of Tanzania, as well as studies on clinical signs in the detection of anaemia and malnutrition. These studies, and two others from Uganda and Bangladesh, are presented in this Supplement to the Bulletin of the World Health Organization.