Adverse events are incidents in which a patient is unintentionally harmed by medical treatment. Awareness while under anaesthetic, deaths during surgery, and missed cases of meningitis are tragic for both patients and staff, and may lead to complaints or litigation. Investigations usually focus on the actions of individual doctors and seldom examine the background to these events.

In a recent case of a patient whose bowel was perforated during surgery, examination of the medical records led to criticism of the surgeon. Only later did it emerge that the operation had been carried out in near darkness because of several equipment and power problems. Adverse events usually originate in a variety of systemic features operating at different levels—the task, the team, the work environment, and the organisation. We present a framework that aims to encompass the many factors influencing clinical practice. It can be used to guide the investigation of incidents, to generate ways of assessing risk, and to focus research on the causes and prevention of adverse outcomes.