The Food and Drug Administration (FDA) is responsible not only for approving drugs but also for monitoring their safety after they reach the market. The complete adverse event profile of a drug is not known at the time of approval because of the small sample size, short duration, and limited generalizability of pre-approval clinical trials. This report describes the FDA's postmarketing surveillance system, to which many clinicians submit reports of adverse drug events encountered while treating their patients. Despite its limitations, the spontaneous reporting system is an extremely valuable mechanism by which hazards with drugs that were not observed or recognized at the time of approval are identified. Physicians are strongly encouraged to submit reports of adverse outcomes with suspect drugs to the FDA, and their reports make a difference. The FDA is strengthening its postmarketing surveillance with access to new data sources that have the potential to further improve the identification, quantification, and subsequent management of drug risk.