The ways in which volume standards are implemented by health services organizations are not clear. Therefore, the authors sought to evaluate the extent of use of volume standards, the purposes for which such standards were developed, and the sources of the standards in a sample of health services organizations. The authors found that volume standards were used widely by accrediting organizations, professional societies, and hospitals in their sample, but almost never by health maintenance organizations. Volume standards were used for ensuring adequate experience among residents, providing guidelines to residency programs, and privileging and credentialing physicians. Expert consensus appeared to be the usual source of volume standards.