Medicare in the United States provides health insurance for Americans aged 65 and older, as well as for people with disabilities identified by the Social Security Administration and those with end-stage renal disease and amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease). Understanding the trajectory of the development over time of this national benefit provides a perspective of the impact of this program on the health and welfare of U.S. citizens and, by extension, the practice of health care providers caring for them.