Calls for the establishment of a “public option,” which emerged during the debate on the Affordable Care Act, have reemerged in this election season. Some proposals base the public option on Medicare, while others on Medicaid. In this article, Wharton professor and LDI Senior Fellow Mark Pauly discusses the likely effects of a public option on private markets, using experience in Medicare Advantage as a guide. Will the public option become the preferred one, sweeping away the private market? Or can the public and private options peacefully coexist?