Administrative law is the law governing the forms, functions, and activities of government agencies. As outlined in the Administrative Procedure Act (APA) the term agency refers to "any 'authority' of the United States government except Congress, the federal courts, territorial governments, and certain military entities." Gary Lawson, Federal Administrative Law, (9th ed. 2021).
Administrative law is divided into three parts: (1) the statutes endowing agencies with powers and establishing rules of substantive law relating to those powers; (2) the body of agency-made law, consisting of administrative rules, regulations, reports, or opinions containing findings of fact, and orders; and (3) the legal principles governing the acts of public agents when those acts conflict with private rights.
The Constitution does not expressly create agencies like it does the Executive, Legislative and Judiciary branches of government. However, it does give Congress the power to "make all Laws which shall be necessary and proper for carrying into Execution" the powers given by the Constitution to the three arms of government. See U.S. Const. art. I Section 8, cl. 18. One of these powers allows Congress, the Executive Branch, and the Federal Courts to create positions in the government, in this case, administrative agencies. Gary Lawson, Federal Administrative Law, (9th ed. 2021).
Most administrative agencies are created by legislative act or statute, sometimes referred to as "organic statutes". These legislative acts create agencies, grant them authority, and define their responsibilities and their limitations. After their creation these agencies then create rules and guidance to assist with carrying out law and policy.