In this brief, we examine the sources of financial pressure on the Medicare program overall and the contributions of Medicare's Parts to that pressure. We then describe the complex array of Medicare financing sources, including their sizes and scopes in relation to gross domestic product, federal spending, and revenue. Because Medicare spending has historically grown faster than national income and is projected to continue doing so, the nation faces increasingly difficult choices between obtaining additional sources of financing that take money away from other public and private uses and reducing Medicare spending. We describe key takeaways that emerge from our review of Medicare’s financing challenges.