Recent reports have identified medical errors as a significant cause of morbidity and mortality among patients. A variety of approaches have been implemented to identify errors and their causes. These approaches include retrospective reporting and investigation of errors and adverse events and prospective analyses for identifying hazardous situations. The above approaches, along with other sources, contribute to data that are used to analyze patient safety risks. A variety of data structures and terminologies have been created to represent the information contained in these sources of patient safety data. Whereas many representations may be well suited to the particular safety application for which they were developed, such application-specific and often organization-specific representations limit the sharability of patient safety data. The result is that aggregation and comparison of safety data across organizations, practice domains, and applications is difficult at best. A common reference data model and a broadly applicable terminology for patient safety data are needed to aggregate safety data at the regional and national level and conduct large-scale studies of patient safety risks and interventions.