Minnesota’s income-tax law permitted taxpayers to deduct from gross income actual expenses incurred for “tuition, textbooks, and transportation” associated with the education of their dependents in elementary and secondary schools. The deduction applied for dependents attending both public and nonpublic schools. Almost all children attending nonpublic schools attended religious schools. Mueller (plaintiff) brought suit against Allen (defendant) in federal court on the ground that the tax law violated the Establishment Clause. The district court and the court of appeals upheld the law as constitutional, and the United States Supreme Court granted certiorari.