The term health maintenance organization was used by the Congress in 1973 when it passed a bill to promote the development of organized systems for the delivery of comprehensive prepaid health care. Prepaid health care organizations were not new—they had been in business for the past 50 years, with the largest and best known being the Permanente (Kaiser) Health Plan. What was new was that Congress defined in fairly specific terms how these organizations should be organized and operated, and provided funds for their development.These organizations would offer individuals and families: (1) an alternative way of paying for health care—a fixed sum of money paid in advance, rather than fee-for-service; (2) an alternative way of receiving care—through a network of providers rather than through solo practitioners; (3) a defined set of benefits, including preventive health services along with education in how to use health services; and (4) a guaranteed delivery system, not just insurance to pay for services. The anticipated results were more efficient use of resources, quality care and lowered cost. Out-patient care would be stressed and in-patient care minimized.The Health Maintenance Act of 1973 recognized two kinds of providers—those practicing in a group practice setting, either as employed staff of the HMO or through a medical group with a contract to provide services to HMO members, and those providers who belonged to an individual practice association which then contracted with the HMO to provide care to HMO members. In the latter model, the physician usually practices out of his own office, is usually paid by the HMO on a fee-for-service basis (although the member prepays to the HMO) and is apt to use health care professionals in the traditional way. In the former model, care is usually provided in a single site or through a group of satellite sites. The medical group is required to use allied health professionals “as appropriate,” and the staff model HMO usually elects to do so.The services prescribed by the HMO Act include physician services, emergency services, out-patient and in-patient hospital services, home health services, and diagnostic laboratory and X-ray, along with services to deal with mental health problems, alcoholism, and drug abuse. Required preventive care includes eye and ear examinations for children, infertility services, family planning, well-child care from birth and periodic health examinations, and pediatric and adult immunizations.Since the Act was passed, over 70 organizations have met the requirements regarding how they are organized and how they operate, and are certified as “qualified HMOs.” With this qualification from the Federal government, the HMO can require the employer to offer the HMO as an alternative to the health insurance plan offered to employees. As more funds are available to develop new HMOs, and as more groups such as large employers develop HMOs with non-governmental assistance, the option will be available to greater numbers of consumers. The opportunity for nurse practitioners to serve as members of HMO health care teams will also expand.In this article, working staff of two qualified HMOs, one a staff model (Georgetown University Community Health Plan), and one a medical group model (North Communities Health Plan, Incorporated) describe how the nurse practitioner functions in the HMO setting.