Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections in infants and young children worldwide, and an important cause of morbidity, hospitalization, and mortality. The infections caused by RSV are seasonal, peaking predictably in the winter months in temperate climates, and in the hottest months and the rainy season in tropical climates. The involvement of the lower respiratory tract, manifest clinically as bronchiolitis or pneumonia, is the hallmark of severe RSV disease. Other indicators of severe disease include requirement for, and duration of, hospitalization, supplemental oxygen, management in an intensive care setting, and mechanical ventilation. Host-related risk factors for severe RSV disease include preterm birth, infection before 6 months of age, chronic lung disease, and congenital heart disease. Environmental risk factors for severe RSV infection include poverty, crowding, exposure to tobacco smoke, and malnutrition. Factors that increase frequency of the infection include young age, multiple gestation, family history of atopy, lack of parental education, household crowding, older school-age siblings, lack of breast feeding, day-care attendance, passive smoke exposure, and discharge from a neonatal intensive care unit between September and December. Recent studies in Europe, North America and Japan have evaluated the number of children affected as well as the medical resources necessary to care for these children. Continuing surveillance is the key to tracking the seasonality, risk factors, morbidity and mortality associated with RSV infection. Epidemiological studies are also the basis for development of appropriate local prevention strategies.