A Health Maintenance Organization (HMO) is a type of Managed Care Organization (MCO) that provides a form of health insurance coverage in the United States that is fulfilled through hospitals, doctors, and other providers with which the HMO has a contract. Unlike traditional indemnity insurance, care provided in an HMO generally follows a set of care guidelines provided through the HMO's network of providers. Under this model, providers contract with an HMO to receive more patients and in return usually agree to provide services at a discount. This arrangement allows the HMO to charge a lower monthly premium, which is an advantage over indemnity insurance, provided that its members are willing to abide by the additional restrictions.

In addition to using their contracts with providers for services at a lower price, HMOs hope to gain an advantage over traditional insurance plans by managing their patients' health care and reducing unnecessary services. To achieve this, most HMOs require members to select a primary care physician (PCP), a doctor who acts as a "gatekeeper" to medical services. PCPs are usually internists, pediatricians, family doctors, or general practitioners. In a typical HMO, most medical needs must first go through the PCP, who authorizes referrals to specialists or other doctors if deemed necessary. Emergency medical care does not require prior authorization from a PCP, and many plans allow women to select an OB/GYN in addition to a PCP, whom they may see without a referral. In some cases, a chronically ill patient may be allowed to select a specialist in the field of their illness as a PCP.

HMOs also manage care through utilization review. The amount of utilization is usually expressed as a number of visits or services or a dollar amount per member per month (PMPM). Utilization review is intended to identify providers providing an unusually high amount of services, in which case some services may not be medically necessary, or an unusually low amount of services, in which case patients may not be receiving appropriate care and are in danger of worsening a condition. 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. When HMOs were coming into existence, indemnity plans often did not cover preventive services, such as immunizations, well-baby checkups, mammograms, or physicals. It is this inclusion of services intended to maintain a member's health that gave the HMO its name. Some services, such as outpatient mental health care, are often provided on a limited basis, and more costly forms of care, diagnosis, or treatment may not be covered. Experimental treatments and elective services that are not medically necessary (such as elective plastic surgery) are almost never covered.

Other methods for managing care are case management, in which patients with catastrophic cases are identified, or disease management, in which patients with certain chronic diseases like diabetes, asthma, or some forms of cancer are identified. In either case, the HMO takes a greater level of involvement in the patient's care, assigning a case manager to the patient or a group of patients to ensure that no two providers provide overlapping care, and to ensure that the patient is receiving appropriate treatment, so that the condition does not worsen beyond what can be helped.

HMOs often shift some financial risk to providers through a system called capitation, where certain providers (usually PCPs) receive a fixed payment per member per month and in return provide certain services for free. Under this arrangement, the provider does not have the incentive to provide unnecessary care, as he will not receive any additional payment for the care. Some plans offer a bonus to providers whose care meets a predetermined level of quality.

Some critics regard HMOs as monopolies that distort the market for health care.