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The Affordable Care Act (“ACA”) is a controversial and historic statute that mandates people make insurance bargains. Unacknowledged is an innovative mechanism ACA uses to select the law that governs those bargains: opt-in federalism. Opt-in federalism—-in which individuals may in part choose between federal and state rules—-is a promising theoretical means to make and choose law. This Article explains why and concludes that the appeal of opt-in federalism is independent of the ACA. Whatever the statute’s constitutional fate, future policymakers should consider opt-in federalist approaches to answer fundamental but exceedingly difficult questions of health and retirement law.