When agencies implement their statutes, administrative law doctrine describes what they do as interpretation. This raises the question of how much deference courts ought to give to such agency interpretations of law. This Article claims, however, that something else is usually going on when agencies implement statutory schemes. Although agencies interpret law, as they must, as an incident to enforce the law, agencies also exercise another power altogether: an interstitial lawmaking, gap-filling, policymaking power, a power that I shall call the “specification power.” This Article aims to advance existing accounts of agency activity and judicial deference by demonstrating that agencies exercise distinct powers of law-interpretation and law-specification when implementing a statutory scheme. Most significantly, it provides a constitutional account for why agencies may exercise this specification power as a formalist matter, even if they cannot have final say over the interpretation of law. If this account is correct, then calls to overturn modern judicial deference may be overblown if agencies are usually exercising their powers not of interpretation, but of specification.