Medicare has four parts: loosely speaking Part A is Hospital Insurance. Part B is Medical Services Insurance. Medicare Part D covers many prescription drugs, though some are covered by Part B. In general, the distinction is based on whether or not the drugs are self-administered but even this distinction is not total. Public Part C Medicare health plans, the most popular of which are branded Medicare Advantage, are another way for Original Medicare (Part A and B) beneficiaries to receive their Part A, B and D benefits; simply, Part C is capitated fee and Original Medicare is fee for service. All Medicare benefits are subject to medical necessity.

The original program included Parts A and B. Part-C-like plans have existed as demonstration projects in Medicare since the early 1970s, but the Part was formalized by 1997 legislation. Part D was enacted by 2003 legislation and introduced January 1, 2006. Previously coverage for self-administered prescription drugs, if desired, was obtained by private insurance or through a public Part C plan (or by one of its predecessor demonstration plans before enactment).

In April 2018, CMS began mailing out new Medicare cards with new ID numbers to all beneficiaries. Previous cards had ID numbers containing beneficiaries' Social Security numbers; the new ID numbers are randomly generated and not tied to any other personally identifying information.