The idea of equality under the law dates back to the foundations of democracy and the ancient Greek word “isonomia.” “Equal justice under law” remains so essential today that it is engraved in the cornice of the Supreme Court building. In 1868, Congress and the states codified this important ideal into the Equal Protection Clause of the Fourteenth Amendment: “No State shall … deny to any person within its jurisdiction the equal protection of the laws.” As the text and history of the Fourteenth Amendment plainly show, the Equal Protection Clause guarantees to all persons — regardless of race, sex, or any other group characteristics — equality under the law, including the legal right to marry the person of one’s choosing. In 2008, California voters passed Proposition 8, restricting the right to marry to opposite‐sex couples only and reversing a California Supreme Court ruling that had authorized same‐sex marriage. Both the federal district court and the Ninth Circuit Court of Appeals ruled that Prop 8 was unconstitutional, for reasons ranging from the violation of a fundamental right to the impropriety of removing rights/benefits once granted. With the case now before the U.S. Supreme Court, the Cato Institute has joined the Constitutional Accountability Center on an amicus brief that focuses on supporting marriage equality under the Equal Protection Clause. Our brief explains that the purpose of the Fourteenth Amendment was not exclusively to address the disparaged rights of former slaves but, as the historical record shows, was intended to be universal in its protection of “any person” within U.S. jurisdiction. The broad and sweeping guarantee of legal equality was understood at the time to secure and protect the equal rights of all individuals, so as to prohibit arbitrary and invidious discrimination. The framers of the Fourteenth Amendment understood state‐sanctioned marriage to be a personal, individual right that must be made available on an equal basis to all. Moreover, the Constitution also protects fundamental rights against state infringement under the substantive liberty provisions of the Fourteenth Amendment. Decades of Supreme Court cases protecting the equal right to marry — without regard to race, being behind on child support payments, or even imprisonment — have been rooted in both the Equal Protection Clause’s guarantee of equality under the law and the Fourteenth Amendment’s broader liberty protections, which converge in securing for all persons an equal right to marry. Prop 8 denies gays and lesbians the liberty to marry the person of their own choosing, places a badge of inferiority on same‐sex couples’ loving relationships and family life (with the full authority of the state behind it), and perpetrates an impermissible injury to these individuals’ personal dignity. It thus directly subverts the principle of equality at the heart of the Fourteenth Amendment, and is an affront to the inalienable right to pursue one’s own happiness that has guided our nation since its founding. We urge the Supreme Court to invalidate Prop 8 as a violation of the foundational guarantee that all persons shall have equality under the law.