Administrative law is largely concerned with two issues. The first is how agencies exercise their power using traditional forms like rulemaking and adjudication. The second is their legitimacy in doing so, which is increasingly under attack in the courts. However, agencies engage in numerous other activities that offer important insights into both these issues. To that end, this Article considers an under-appreciated agency activity: the practice of appearing as amicus curiae in the federal courts. Agencies file scores of amicus briefs each year, all seeking to influence the development of the law outside of the traditional means of rulemaking and adjudication. This Article’s primary objectives are to build a positive account of agency amicus behavior, including a typology of such behaviors, and to evaluate those behaviors through the normative dimensions of administrative law. As developed herein, this exploration offers deep engagement with some of the most pressing issues of administrative law today, including the scope of agencies’ interpretive authority, implications of a strong view of the unitary executive, and access to justice. As the Article concludes, agency amici fill important roles both in ordinary administrative law and in supporting the constitutional framework.