What does it mean to say that an administrative agency is a source of law? Agencies constantly generate what our society regards as law, making and remaking legal obligations at a frenetic pace. Yet most of the rules agencies make lack the force of law and recognizing those that have this force is often an exceptionally complicated task. It is a task, in fact, that courts are usually not suited to performing, at least not very well. In this Article, I argue that the nature of contemporary legislative language and our turn toward informal agency processes have combined to render the bulk of lawmaking in our society unrecognizable within our conventional pictures of legality. If legal rules are, as most positivists maintain, simply “exclusionary reasons” by which legal actors guide their conduct, agency rules that are never enacted as “law” are becoming more and more of our law’s content. Informal processes like notice and comment were neither the beginning nor the culmination of our legal system’s departure from traditional, standardized forms of law. But they may be the best evidence of a highly plastic concept of law that is ascendant today.