 All four Parts of Medicare—A, B and C, and D—are administered by private companies under contract to the Centers for Medicare and Medicaid Services (CMS).Almost all these companies are insurance companies, except for those that administer most Medicare Advantage and other Part C health plans, which are not -- strictly speaking -- insurance policies.A plurality of Medicare Advantage and other Part C health plans are administered (CMS uses the term "sponsored") by non-profit integrated health delivery systems and their spin offs.