Bronchiolitis is the most common disease of the respiratory tract during the first year of life, and occurs in annual epidemics in winter. The etiology is viral, and respiratory syncytial virus (RSV) is the commonest agent. Respiratory symptoms remain generally mild, and treatment just supportive and at home. Certain infants are at high risk of severe illness (age less than 3 months, preterm birth, neonatal respiratory disease, bronchopulmonary dysplasia, underlying chronic diseases), and require hospitalisation. Most treatments are of unproved (corticosteroids), or limited benefit (inhaled bronchodilators, antibiotics). Chest physiotherapy is indicated in case of bronchial secretion. Vaccine are not now available, but prophylaxis with human RSV immunoglobulin or monoclonal antibodies has to be considered for children at increased risk for severe disease.