Bronchiolitis is the most common lower respiratory track infection in infancy. The pathophysiology consists of acute inflammation, edema, increased mucus production, and bronchospasm. The condition starts as rhinitis followed by tachypnea, wheezing, cough, crackles, use of accessory muscles, and/or nasal flaring a few days later. Respiratory syncytial virus (RSV) is the cause in 50–70% of cases, with the highest incidence occurring between December and March. Almost all infants will have an RSV infection by the time they reach 2 years of age, although infection does not grant permanent or long-term immunity. Reinfections are common throughout life. Other viruses causing bronchiolitis are human metapneumovirus, influenza, adenovirus, and parainfluenza. The American Academy of Pediatrics does not recommend routine chest radiographs or laboratory testing on patients with mild to moderate bronchiolitis. Treatment is primarily supportive. Racemic epinephrine, nebulized beta-agonist may be helpful in certain cases but should not be routinely used. Steroids have not been shown to be beneficial.