In countries that lack a single canonical text, the "constitution" is defined to include all those laws that perform the constitutive functions of creating governmental institutions and conferring rights on individuals. The British Constitution, for example, is generally thought to include a variety of constitutive statutes, such as Magna Carta, the Parliament Acts, and the Human Rights Act. This article proposes a thought experiment: What if we defined the U.S. constitution by function, rather than by form? Viewed from this perspective, "the Constitution" would include not only the canonical document but also a variety of statutes, executive materials, and practices that structure our government. It would include, for example, the Judiciary Act of 1789, the Administrative Procedure Act, the Executive Orders establishing Presidential control of administrative rulemaking, and the non-statutory rules that govern voting in the House and Senate and the structure of our major political parties. These sorts of extra-canonical materials perform the basic constitutional functions of constituting the government and conferring rights on individuals. What they lack is a third characteristic shared by some (but not all) constitutions: formal entrenchment against legal change. Entrenchment has become central to the American conception of a constitution, but I propose here to decouple the entrenching function from the constitutive function for purposes of constitutional analysis. This approach offers a relatively simple answer to one of the most important problems in constitutional theory: How do we explain the evident fact that the structure of our government and the rights of the people have changed pervasively since the Founding, in ways that are simply not reflected in Article V amendments to the canonical text? The answer is that the constitutional order can change in this way because most of it was never entrenched in the canonical text to begin with. Most of the salient changes - the growth of the administrative state, the proliferation of individual entitlements - are changes to our "constitution outside the constitution" that are neither mandated nor forbidden by the canonical document. Because I do not view these changes as any more entrenched than the arrangements they replaced, I do not need to develop any complex and contestable theory of "higher lawmaking" to set these changes apart from other "ordinary" legislation. The functional account of constitutionalism offered here also has implications for constitutional doctrine and scholarship. My account tends to undermine doctrinal prescriptions grounded in a sharp dichotomy between constitutional and statutory claims. Such prescriptions would include arguments that the federal courts' irreducible jurisdiction under Article III should be defined by their power to hear constitutional claims, or that the federal civil rights statute, 42 U.S.C. § 1983, should be interpreted to exclude claims under federal statutes and regulations. Likewise, recognizing the constitutive functions of statutes and regulations suggests that basic constitutional values - such as federalism or concern for individual rights - are just as relevant to statutory construction as they are to interpreting the constitutional text. Finally, the functional account suggests a broader set of concerns for constitutional law teaching and scholarship; in particular, it suggests that constitutional scholars should be concerned with the institutional design of constitutive statutes, treaties, and regulations rather than solely focused on interpreting the canonical text.