Respiratory syncytial virus (RSV) and the parainfluenza viruses (PIVs) are the most important causes of acute lower respiratory illness (LRI) in infants and children under 6 years of age. These enveloped viruses are members of the paramyxovirus family. They infect cells in the epithelium lining the trachea and intrapulmonary airways, and cause croup, bronchitis, bronchiolitis, and bronchopneumonia. RSV causes annual midwinter to early spring outbreaks of respiratory disease in temperate climates; epidemics are heralded by the appearance of increased numbers of cases of bronchiolitis, primarily in children under 2 years of age. PIV serotypes 1 and 2 cause epidemics of croup in the fall months. Infections with PIV serotype 3 can occur in an endemic pattern throughout the year, or may occur as outbreaks, usually in the fall or spring. Croup and bronchiolitis are the most common syndromes of PIV-3 LRI. Infection with these viruses induces short-lived partial resistance to reinfection, but the human host remains susceptible to reinfection with these agents throughout life. While antibody in respiratory secretions is related most directly to resistance to reinfection, cell-mediated immune responses are crucial for limitation and termination of established infection. Current research efforts are directed at more thorough characterization of the developing host immune response to individual viral antigens, and to development of methods for immunization using specific virion peptides. Recently, antiviral therapy has become available for serious RSV infection in young infants.