Lemon v. Kurtzman is a landmark Supreme Court case from 1971 that established the 'Lemon Test' to determine whether government actions violate the Establishment Clause of the First Amendment, which prohibits the government from endorsing or promoting religion. The case involved a Pennsylvania law that provided financial aid to non-public schools, and the court ruled that such aid must not advance or inhibit religion, must have a secular purpose, and must avoid excessive entanglement with religion. This decision plays a critical role in maintaining the separation of church and state.