This article discusses Rush-Presbyterian-St. Luke's Medical Center's approach to assessing and preventing errors in care and promoting patient safety. The word error is applied to all kinds of events, including adverse occurrences, negligence, and malpractice. Thus confusion exists among those analyzing the causes of adverse events. A patient safety committee standardized the definition of medical error and developed a taxonomy for error as a prelude to efforts at error reduction. It identified three levels or layers that can represent a train of events culminating in an undesired outcome: error, treatment failure, and adverse event. This discussion is offered in the interest of clarifying some of the issues.