In its 2000 report, To Err is Human, the Institute of Medicine concluded that between 44,000 and 98,000 deaths per year occur in United States hospitals as a result of error. These data have resulted in calls for further research, regulatory interventions, third-party payer involvement, and health care organization initiatives to improve this situation. Studies of pediatric inpatients suggest that medication-related harm occurs at a rate as high as 11.1 per 100 admissions, and hospital-related harm occurs in high risk neonatal ICUs at a rate of 74 per 100 admissions. This article discusses differences between error and harm, methods used to measure harm, and available evidence that identifies the incidence of adverse events in pediatric inpatients and outpatients.