Acute lung injury (ALI) is a syndrome of severe acute respiratory failure defined by a constellation of clinical criteria. The exact incidence of ALI is not known, but it generally occurs in the setting of acute severe illness. The course of ALI after onset is quite variable, but outcome is associated with risk factor, age, and comorbidity. Survival from this syndrome has improved over time. Survivors often are impaired after hospital discharge but tend to improve over time; however, ALI does confer a significant additional burden on survivors with regard to pulmonary function and health-related quality of life.