An HMO doctor is a healthcare professional who treats patients based on a prepaid basis. When a person joins and HMO (health maintenance organization) they make preset, regular monthly contributions to a health care organization (the HMO) in exchange for the security of knowing their treatment is covered (already paid for) by their HMO insurance plan. This means they will not incur any out-of-pocket expenses. An HMO doctor is a doctor that has agreed to participate in this sort of healthcare plan. HMO members must seek the care of an HMO doctor, a doctor that participates in the plan. An HMO should not be confused with a PPO. While they are both healthcare plans, they differ in certain ways. A PPO is a group of doctors that only treat a specific group of people. PPO member also pay for their medical care as it arises, the price of which is pre-negotiated between the healthcare provider and the PPO sponsors.