A health maintenance organization (HMO) is an organization that provides or arranges managed care for health insurance, self-funded health care benefit plans, individuals and other entities in the United States as a liaison with health care providers (hospitals, doctors, etc.) on a prepaid basis. The Health Maintenance Organization Act of 1973 required employers with 25 or more employees to offer federally certified HMO options if the employer offers traditional healthcare options. Unlike traditional indemnity insurance, an HMO covers care rendered by those doctors and other professionals who have agreed by contract to treat patients in accordance with the HMO's guidelines and restrictions in exchange for a steady stream of customers. HMOs cover emergency care regardless of the health care provider's contracted status.

HMOs require members to select a primary care physician (PCP), a doctor who acts as a "gatekeeper" to direct access to medical services. PCPs are usually internists, pediatricians, family doctors, or general practitioners (GPs). Except in medical emergency situations, patients need a referral from the PCP in order to see a specialist or other doctor, and the gatekeeper cannot authorize that referral unless the HMO guidelines deem it necessary.