Medicare buy-in policies gained prominence as potential incremental health reforms in the mid-1990s, after the Clinton administration's more ambitious health reform plan failed. Such proposals from that era would have created a guaranteed source of health insurance older adults could buy, before becoming eligible for Medicare at age 65. In this brief, we discuss the potential merits of a Medicare buy-in policy that coexists with the ACA. We first show how older adults are at substantial risk of high health care spending, making health insurance coverage critical to their financial security. We then show how uninsurance rates have varied with age before and after ACA implementation. We highlight some main findings from a recent report that estimated the health insurance coverage and spending effects of Medicare buy-in policies and discuss what buy-in policies could achieve if they were implemented alongside the ACA.