The single-payer health insurance proposal known widely as Medicare for All (M4A) cannot be enacted without first answering certain questions. Foremost among these is whether the public would support shifting more than $32 trillion in M4A’s first 10 years from private health spending, over which consumers retain some discretion, to federal health spending, over which consumers do not. A related open question is whether the federal government can adequately finance this amount of spending without triggering significant adverse economic effects. Other unanswered questions include M4A’s effects on health providers, the prescription drug market, and private health insurance. M4A would add further to national health cost growth unless provider reimbursements are cut more sharply than lawmakers have been willing to do historically. Yet the consequences of enacting such payment cuts simultaneously with a substantial increase in health service demand are unpredictable.