An HMO is a type of health plan. With an HMO, you can only see doctors and go to hospitals that are in your health plan's network, unless it's an emergency. If you go out of network, your care may not be covered by your health plan, and you will have to pay the bill. You also usually need a referral from your primary care provider (PCP) to see another doctor, such as a specialist. If you have an HMO, it's important to always check to see that you are using an in-network provider before getting health services.