The enormous and rapid changes taking place in health care delivery are forcing institutions, organizations, and individual providers of care to reexamine their relationships and create new methods of assuring both quality of care and safety for consumers. One result has been the emphasis on systematic methods of credentialing, privileging, and measuring and evaluating an individual's qualifications and actual performance. This article discusses the credentialing and privileging processes that are either in place or evolving for physicians in the managed care health care systems. With the continued evolution of managed care in this country, advanced practice nurses may need to expand their credentialing process. This article provides information on the evolutionary events leading to credentialing and privileging, current and anticipated credentialing measures, sources of measurements, medicolegal issues, and suggestions for expansion of nurse credentialing. A comprehensive credentialing system will provide additional evidence of the nursing profession's capacity to play a major role in the reformed health care system.