Lemon v. Kurtzman, 403 U.S. 602 (1971), was a case argued before the United States Supreme Court. The court ruled in an 8–0 decision that Pennsylvania's Nonpublic Elementary and Secondary Education Act (represented through David Kurtzman) from 1968 was unconstitutional and in an 8–1 decision that Rhode Island's 1969 Salary Supplement Act was unconstitutional, violating the Establishment Clause of the First Amendment. The act allowed the Superintendent of Public Schools to reimburse private schools (mostly Catholic) for the salaries of teachers who taught in these private elementary schools from public textbooks and with public instructional materials.

The Court applied a three-prong test, which became known as the Lemon test (named after the lead plaintiff Alton Lemon), to decide whether the state statutes violated the Establishment Clause.

The Court held that the Establishment Clause required that a statute satisfy all parts of a three-prong test:

In the 1985 case Wallace v. Jaffree, the Supreme Court stated that the effect prong and the entanglement prong need not be examined if the law in question had no obvious secular purpose. In Corporation of Presiding Bishop of Church of Jesus Christ of Latter-day Saints v. Amos (1987) the Supreme Court wrote that the purpose prong's requirement of a secular legislative purpose did not mean that a law's purpose must be unrelated to religion, because this would amount to a requirement, in the words of Zorach v. Clauson, 343 U. S. 306 (1952), at 314, "that the government show a callous indifference to religious groups." Instead, "Lemon's 'purpose' requirement aims at preventing the relevant governmental decisionmaker—in this case, Congress—from abandoning neutrality and acting with the intent of promoting a particular point of view in religious matters." The Supreme Court further explained in McCreary County v. American Civil Liberties Union (2005) that" "When the government acts with the ostensible and predominant purpose of advancing religion, it violates that central Establishment Clause value of official religious neutrality, there being no neutrality when the government’s ostensible object is to take sides."

The act at issue in Lemon stipulated that "eligible teachers must teach only courses offered in the public schools, using only materials used in the public schools, and must agree not to teach courses in religion." Still, a three-judge panel found 25% of the State's elementary students attended private schools, about 95% of those attended Roman Catholic schools, and the sole beneficiaries under the act were 250 teachers at Roman Catholic schools.

The Court found that the parochial school system was "an integral part of the religious mission of the Catholic Church", and held that the Act fostered "excessive entanglement" between government and religion, thus violating the Establishment Clause.

Held: Both statutes are unconstitutional under the Religion Clauses of the First Amendment, as the cumulative impact of the entire relationship arising under the statutes involves excessive entanglement between government and religion.

The Lemon test was modified, according to the First Amendment Center, in the 1997 case Agostini v. Felton in which the U.S. Supreme Court combined the effect prong and the entanglement prong. This resulted in an unchanged purpose prong and a modified effect prong. As the First Amendment Center notes, "The Court in Agostini identified three primary criteria for determining whether a government action has a primary effect of advancing religion: 1) government indoctrination, 2) defining the recipients of government benefits based on religion, and 3) excessive entanglement between government and religion."