After decades of debate, the lines of distinction between textualism and purposivism have been carefully drawn with respect to the judicial task of statutory interpretation. Far less attention has been devoted to the question of how executive branch officials approach statutory interpretation. While scholars have contrasted agencies’ interpretive practices from those of courts, they have not yet developed a theory of agency statutory interpretation. This Article develops a purposivist theory of agency statutory interpretation on the ground that regulatory statutes oblige agencies to implement the statutes they administer in that manner. Regulatory statutes not only grant powers but also impose a duty on agencies to carry out those powers in accordance with the principles or purposes the statutes establish. To comply with that duty, agencies must develop a conception of the purposes that the statute requires them to pursue and select a course of action that best carries forward those purposes within the means permitted by the statute; in short, agencies must take a purposivist approach. Moreover, this Article argues that agencies’ institutional capacities—a familiar constellation of expertise, indirect political accountability, and ability to vet proposals before adopting them—make them ideally suited to carry out the task of purposive interpretation. Understanding agency interpretation as purposive by statutory design has significant implications for long-standing debates. First, it suggests that the focus of judicial review should be on the agency’s specification of the statute’s purposes and chosen means to implement those purposes, questions that are not squarely addressed by the Chevron doctrine. Second, by providing an account of the character of the agency’s statutory duties, this analysis helps to distinguish appropriate from inappropriate political and presidential influences on the agency. Finally, investigating the debate between purposivism and textualism beyond the courts exposes a renewed promise—and project—for purposivism.