Enshrined in the U.S. Constitution and an integral element of democratic self‐governance generally is the fundamental right of all people to be treated equally by their government — to receive “equality under the law” in both procedure and substance. Yet at least one important federal law, with cascading effects on many others, denies that equal protection on the basis of sexual orientation: The Defense of Marriage Act, signed into law by President Clinton in 1996, which contains a provision, Section 3, that defines “marriage” in all federal statutes as a legal union between one man and one woman. This definitional detail affects more than 1000 federal provisions — from tax returns and veterans’ benefits, to Social Security and healthcare, to housing and immigration. That is, federal law views lawfully married same‐sex couples (who were married in one of the states or countries that recognizes these unions) differently from lawfully married opposite‐sex couples. Aside from treating individuals adversely on the basis of their sexual orientation, Section 3 also imposes discriminatory costs on all sorts of private employers and contractors, due to the complex operation of federal employee benefits law — to give one example of DOMA’s reach. Another example comes in tax law: Edith Windsor, the plaintiff in this case, had been in a loving, committed relationship with Thea Spyer for 42 years when they legally married in Canada in 2007. When Spyer passed away in 2009, Windsor not only had to deal with the grief of losing her partner, but was forced to pay federal estate taxes totaling $363,053 because Section 3 prevents the IRS from recognizing Windsor as a surviving spouse. When Windsor sued to get her money back, two lower federal courts found Section 3 unconstitutional and ordered the requested tax refund. The Second Circuit Court of Appeals (based in New York) held that a law targeting gays and lesbians is subject to heightened scrutiny as wrongful discrimination, which Section 3 cannot survive. The case is now before the Supreme Court — through a complicated procedural maneuver because the federal government stopped defending DOMA in the courts — and Cato has joined the Constitutional Accountability Center on a brief that focuses on the Fifth Amendment’s equal protection guarantee (part of the liberty protected by that amendment’s Due Process Clause). The principle of “equal protection” requires the federal government to treat all similarly situated persons alike and prohibits it from drawing distinctions between individuals based solely on differences that are irrelevant to legitimate governmental objectives. Under any standard of review, DOMA’s sweeping discrimination contravenes this constitutional guarantee, which the Supreme Court has consistently ruled to secure equality under the law and forbid invidious discrimination. The Fifth Amendment forbids the federal government from enacting laws arbitrarily singling out a class of individuals for disfavored legal status. The government thus cannot discriminate among individuals who are lawfully married on the basis of their sexual orientation. DOMA was born out of animosity towards homosexuals and does nothing to further the supposed objectives regarding marriage put forward by the government. We urge the Supreme Court to invalidate Section 3 as an affront to the Constitution’s guarantee of equality under the law and inconsistent with the rights and freedoms extending to all who live in the United States.