Medicare Accountable Care Organization (ACOs) are groups of doctors, hospitals, and other health care providers who voluntarily form partnerships to have collective responsibility for the quality and total cost of care delivered to their patients. ACOs are not health plans and you cannot “enroll” in them. Medicare may assign you to an ACO if your primary care doctor is in one, but you are still free to seek services from any Medicare provider inside or outside of the ACO. If you want Medicare to assign you to an ACO based on a certain doctor, you can enter his or her name through your account information on the Medicare website. For more information about ACOs, see the Accountable Care Organization page on the Medicare.gov website.