This Agency for Healthcare Research and Quality (AHRQ)–supported study prospectively observed intensive care units over a 1-year period and analyzed captured incidents. Investigators used a variety of methods, including direct observation, to examine nearly 1500 patient-days. The incident rate per 1000 patient-days was greatest for the category defined as serious errors followed by that of preventable adverse events. Discussion provides details of the clinical patient characteristics, the range in severity of incidents, and specific examples and frequencies of the defined event types. The authors conclude that, while critical care settings offer vital services in treating patients, the setting also carries noted risks for adverse events and errors, and it is important to promote ongoing improvement efforts.