A health maintenance organization (HMO) is a type of managed care organization (MCO) that provides a form of health care coverage in the United States that is fulfilled through hospitals, doctors, and other providers with which the HMO has a contract. The Health Maintenance Organization Act of 1973 required employers with 25 or more employees to offer federally certified HMO options.[1] Unlike traditional indemnity insurance, an HMO covers only care rendered by those doctors and other professionals who have agreed to treat patients in accordance with the HMO's guidelines and restrictions in exchange for a steady stream of customers.

Most 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). Absent a medical emergency, 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.

"Open access" HMOs do not use gatekeepers - there is no requirement to obtain a referral before seeing a specialist. The beneficiary cost sharing (e.g., co-payment or coinsurance) may be higher for specialist care, however.[2]

HMOs also manage care through utilization review. That means they monitor doctors to see if they are performing more services for their patients than other doctors, or fewer. HMOs often provide preventive care for a lower copayment or for free, in order to keep members from developing a preventable condition that would require a great deal of medical services.

In recent years most HMO’s have dropped having a primary care physician / gatekeeper for a few reasons, mostly due to customer dissatisfaction an inconvenience.