Prescriptive standards of clinical conduct--practice guidelines--have proliferated throughout medicine over the past decade. Practicing physicians are confronted with a plethora of guidelines developed for different purposes by a diverse body of public and private organizations. We review factors contributing to the growth of guidelines, their desirable features, and consequences, legal and otherwise, of implementing guidelines. Few studies have examined whether, and under what conditions, guidelines are effective in changing physicians' practices and patients' health. Nonetheless, expectations for guidelines remain high because they are one of the only instruments of health care reform that promises to improve the quality of care while reducing overall health care costs. Efforts to develop guidelines are likely to continue unabated for the foreseeable future. Additional research comparing different methods of developing and disseminating guidelines is needed.