When modern American courts assess the constitutionality of a statute, they often investigate the possibility that the enacting legislature had hidden purposes that make the statute invalid. For most of our history, though, courts shied away from such inquiries -- not because the Constitution was thought to impose no purpose-based restrictions on legislative power, but because those restrictions were not thought to lend themselves to much judicial enforcement. This Article provides a comprehensive history of changes over time in judicial review of legislative purpose. Given the increasing prominence of purpose tests in modern constitutional adjudication, this history is important in its own right. But it also sheds light on other topics of interest to modern lawyers, including the proper interpretation of various seminal precedents, the famously murky doctrine of "unconstitutional conditions," and the ways in which uncodified norms of judicial practice can affect the glosses that courts put on the Constitution's text.