A 1971 Supreme Court decision interpreting the Establishment Clause of the First Amendment that struck down state statutes providing aid to parochial schools and established a three-part test requiring (1) a secular legislative purpose, (2) a primary effect that neither advances nor inhibits religion, and (3) avoidance of excessive government entanglement with religion. Its framework restricts state and local financial support for religious institutions and remains a central precedent shaping church–state separation and judicial review of government actions affecting religion.