Almost all the Constitution’s provisions apply to governments, state and federal, and not directly to private people. But the legal rights of private people are protected by the government, which raises the question whether exercises of those rights are ever subject to constitutional rules for that reason. A standard feature of American constitutional law, the state action principle, holds that in general the decisions of private people in the exercise of their legal rights are not attributed to the government for purposes of the Constitution, even though the government’s coercive power supports those rights. The state action principle has long been a matter of controversy, and several important contemporary scholars of constitutional law have criticized it, suggesting that it rests on a failure to understand that private rights rest on government coercion and that it interferes with the proper implementation of some important substantive constitutional rules. This article defends the state action principle, arguing that it is conceptually coherent and reflects a vision of the Constitution that, although subject to debate as a normative matter, has much to be said for it. Rather than resting on a failure to see public power behind private rights, the principle is founded on the idea that private people, when they exercise private rights, are principals who are entitled to act on their own behalf. Government officers and institutions, by contrast, are agents, acting on behalf of others. That distinction, not the presence of government coercion, supports the different treatment of private people exercising state-supported private rights and government actors exercising government power. The article also argues that the state action principle does not undermine the constitutional norms that protect particular forms of liberty like free expression or that forbid certain forms of discrimination, as the critics suggest. Rather, the state action principle fits those protections for liberty and equality into a constitutional system in which the vast bulk of legal rules, including in particular the rules that give private people control over material resources, are found in the non-constitutional law and not the Constitution itself.