This chapter considers the remarkable diversity of American religion from the beginning: the constitutionalizing of religion, the reasons for delayed implementation of the First Amendment religion clauses, the evolution of free exercise and establishment clause doctrine, and the ongoing difficulty of defining religion for US law. What makes US regulation of religion stand out among national legal orders is the dual commitment to federalism and to disestablishment. With a low ‘statism’ and a strong commitment to equality—theological, as well as political—academic expertise has little purchase on the national mind. Religion is what the people say it is. That is a very old story in the US.