Borrowing from one domain to promote ideas in another domain is a staple of constitutional decisionmaking. Precedents, arguments, concepts, tropes, and heuristics all can be carried across doctrinal boundaries for purposes of persuasion. Yet the practice itself remains underanalyzed. This Article seeks to bring greater theoretical attention to the matter. It defines what constitutional borrowing is and what it is not, presents a typology that describes its common forms, undertakes a principled defense of borrowing, and identifies some of the risks involved. The authors' examples draw particular attention to places where legal mechanisms and ideas migrate between fields of law associated with liberty, on the one hand, and equality, on the other. They finish by discussing how attentiveness to borrowing may illuminate or improve prominent theories of constitutional lawmaking.