Arizona grants income tax credits for contributions made to school tuition organizations ("STOs"). STOs must use these donations for scholarships that allow students to attend private schools. This statutory scheme broadens the educational opportunities for thousands of students by enabling them to attend schools they would otherwise lack the means to attend. Although the Ninth Circuit acknowledged that increasing educational opportunities is a valid secular purpose for a legislative act, it found that the tax credit program nonetheless violates the Establishment Clause because many of the STOs — as it happens, a decreasing majority — provide scholarships for students to attend parochial schools. Cato, in a brief joined by four education reform groups, urges the Supreme Court to overturn the Ninth Circuit's decision because it was based on faulty reasoning: It equated the private and voluntary choices of individuals who donate to religious STOs with state sponsorship of religion. The lower court also made the dubious assertion that Arizona parents feel pressured to accept scholarships to religious schools, in spite of the fact that the share of STO scholarships available for use at secular schools is almost twice as large as the share of families actually choosing secular schools. Moreover, the tax credit scheme is indistinguishable from similar charitable tax deduction programs that the Court has previously held to pass constitutional muster. We urge the Court to reaffirm its longstanding jurisprudence — especially the 2002 school-choice case, Zelman v. Simmons-Harris — whereby instances of "genuine and independent choice" are insulated from Establishment Clause challenge. Far from being an impediment to parental freedom, the autonomy Arizona grants to taxpayers and STOs is ultimately essential to it. More generally, should the lower court's opinion be allowed to stand, the progress made to broaden the educational opportunities of students across the country will be stifled.