Emergency department (ED) patients are particularly vulnerable to adverse events, and a prior study of closed malpractice claims implicated systems factors such as poor teamwork in adverse patient outcomes. This study used root cause analysis of incident reports to identify the types and causes of errors and unanticipated events in the ED. Incidents included poor communication and teamwork, particularly with other departments, but medication errors and diagnostic errors were also noted. The authors recommend that organizations integrate the ED into hospital-wide safety improvement efforts.