In exchange for low co-payment (or sometimes no co-pay at all), low premiums and minimal paperwork, an HMO requires that you only see its doctors, and that you get a referral from your primary care physician before you see a specialist. If you can still pick up the phone, you'll probably need to get clearance before you can visit the emergency room.

An HMO may have central medical offices or clinics (such as those used by Kaiser Permanente), or it may consist of a network of individual practices. In general, you must see HMO-approved physicians or pay the entire cost of the visit yourself. HMOs have the best reputation for covering proventive care services and health improvement programs.