The strategy of Integrated Management of Childhood Illness (IMCI) aims to reduce child mortality and morbidity in developing countries by combining improved management of common childhood illnesses with proper nutrition and immunization. The strategy includes interventions to improve the skills of health workers, the health system, and family and community practices. This article describes the experience of the first countries to adopt and implement the IMCI interventions, the clinical guidelines dealing with the major causes of morbidity and mortality in children, and the training package on these guidelines for health workers in first-level health facilities. The most relevant lessons learned and how these lessons have served as a basis for developing a broader IMCI strategy are described.