Respiratory syncytial virus (RSV) is one of the most common respiratory pathogens in infants and young children worldwide. Almost all children are infected at least once by the age of 2 years. The clinical manifestations vary depending on age, health status, and whether the infection is primary or secondary. Most RSV-infected children experience upper respiratory tract symptoms and 20% to 30% develop lower respiratory tract disease (e.g., bronchiolitis and/or pneumonia). RSV is the most common cause of lower respiratory tract infection in children younger than 1 year. Most children who have been previously healthy have mild lower respiratory tract infections. However, prematurity, chronic lung disease, congenital heart disease, and primary or secondary immune deficiencies are risk factors for severe RSV infections. Diagnosis can often be clinical. Diagnostic testing is not routinely recommended. The treatment of upper and lower respiratory tract infections is generally supportive. Ribavirin can be used in patients with immune deficiency but is not routinely recommended. There is not a routine vaccine for RSV. The RSV-specific humanized mouse monoclonal antibody "palivizumab" is particularly safe and effective for prophylaxis in high-risk groups.