The Free Exercise Clause is the accompanying clause with the Establishment Clause of the First Amendment to the United States Constitution. The Establishment Clause and the Free Exercise Clause together read:

In 1878, the Supreme Court was first called to interpret the extent of the Free Exercise Clause in Reynolds v. United States, as related to the prosecution of polygamy under federal law. The Supreme Court upheld Reynolds' conviction for bigamy, deciding that to do otherwise would provide constitutional protection for a gamut of religious beliefs, including those as extreme as human sacrifice. The Court said: "Congress cannot pass a law for the government of the Territory which shall prohibit the free exercise of religion. The first amendment to the Constitution expressly forbids such legislation." Of federal territorial laws, the Court said: "Laws are made for the government of actions, and while they cannot interfere with mere religious beliefs and opinions, they may with practices."