The incorporation of the First Amendment establishment clause in the landmark case of Everson v. Board of Education has impacted the subsequent interpretation of the separation of church and state in regard to the state governments. Although upholding the state law in that case, which provided for public busing to private religious schools, the Supreme Court held that the First Amendment establishment clause was fully applicable to the state governments. A more recent case involving the application of this principle against the states was Board of Education of Kiryas Joel Village School District v. Grumet (1994).

What landmark case has impacted all subsequent interpretations of the separation of church and state in regard to state governments?