In Arizona, residents were entitled to tax credits for contributions made to school tuition organizations (STOs). STO contributions were then distributed as scholarships to private-school students at both religious and non-religious schools. A group of Arizona taxpayers (plaintiffs) brought an action against Kathleen Winn (defendant), the Director of the Arizona Department of Revenue, in federal district court. The plaintiffs challenged the STO tax credit, alleging that the tax credit amounted to the State of Arizona using tax revenue to pay tuition at religious schools in violation of the Establishment Clause. The district court dismissed the action for failure to state a claim, but the appellate court reversed and held that the plaintiffs stated a claim for a violation of the Establishment Clause. The United States Supreme Court granted certiorari.