The Tax Relief and Health Care Act of 2006 mandated that the Office of Inspector General (OIG) report to Congress the incidence of "never events" among Medicare beneficiaries, payment by Medicare for services in connection with such events, and the process used to identify events and deny payments. This report addresses that mandate by providing a descriptive analysis of the key issues to understanding hospital-based adverse events. The report is focused around discussion of seven critical issues that are explored in detail. Of note, OIG expanded the study of never events to the broader topic of adverse events in their analysis.