Health Maintenance Organizations, or HMOs, coordinate with doctors, medical facilities and insurance companies to organize your care and control costs. HMOs have a fixed network of doctors, and usually provide all health services. If you need specialist treatment, you must get a referral from your primary care physician. HMOs generally do not cover out-of-network doctors unless it's an emergency. HMOs use a co-pay system, so you pay only a small amount of out-of-pocket money per doctor's visit, encouraging you to take preventive care.