This article summarizes the epidemiology of medical errors. The authors provide findings from benchmark studies to describe the prevalence and consequences of errors in the hospital setting. They also explore similar data for the outpatient setting, which are limited. Following this background, they discuss types of error, including complications from drug treatment, therapeutic mishaps, and diagnostic failures. The authors illustrate the number of preventable adverse events and those resulting in permanent disability. They explain a strategy to prevent errors by identifying individuals at high risk, such as elderly patients or those undergoing planned high-risk surgical procedures. They conclude by expressing the challenges in error reporting and emphasizing the fact that risk is not homogenous. This article is from a British Medical Journal special issue on patient safety.