The Equal Protection Clause of the Fourteenth Amendment, which reads,  “[N]o State shall . . . deny to any person within its jurisdiction the equal protection of the laws,” is an invaluable tool for groups that experience discrimination.  In addition to binding the states, it also applies to the federal government through the Due Process Clause of the Fifth Amendment. To establish an Equal  Protection violation, a plaintiff must prove purposeful discrimination directed at an identifiable or suspect class. Classes are defined by an individual’s characteristics—for example, sex, sexual orientation, or race—and those classes determine the level of scrutiny received under the Equal Protection Clause.

Part II of this article provides an overview of the principles of constitutional equal protection, discusses the three levels of judicial scrutiny and their corresponding triggers, and briefly addresses potential alternatives. Part III considers sex-based classifications under the federal and state constitutions. It first describes the federal intermediate scrutiny test and details common and contentious areas of the law. It also addresses the extent to which United States v.  Virginia, Nguyen v. INS, and Sessions v. Morales-Santana altered the framework for analyzing sex-based classifications. Part III next discusses the standards of review that states apply in sex-based discrimination claims.  Part IV addresses discrimination based on sexual orientation under both the  Equal Protection Clause and the Due Process Clause. This section begins with an overview of Romer v. Evans, which established the applicability of rational basis review for sexual orientation-based classifications. The section then covers  Lawrence v. Texas—a landmark case in which the Supreme Court determined that homosexual people’s right to liberty under the Due Process Clause gives them the right to engage in sexual conduct without interference from the government. Next, the section looks at how district and circuit courts have used the  Romer standard10 to limit the rights granted in that decision and summarizes the pre-Obergefell circuit split on same-sex marriage and the resulting Supreme  Court decisions expanding the right to marry to all same-sex couples in the 2013  Windsor decision12 and the 2015 Obergefell decision. Finally, Part IV explores state constitutions’ varying levels of scrutiny of sexual orientation classifications.