The US Constitution mentions only three branches of government: legislative, executive, and judicial (Articles I, II, and III). There is no mention of agencies in the Constitution, even though federal agencies are sometimes referred to as “the fourth branch of government.” The Supreme Court has recognized the legitimacy of federal administrative agencies to make rules that have the same binding effect as statutes by Congress.
Most commentators note that having agencies with rule-making power is a practical necessity: (1) Congress does not have the expertise or continuity to develop specialized knowledge in various areas (e.g., communications, the environment, aviation). (2) Because of this, it makes sense for Congress to set forth broad statutory guidance to an agency and delegate authority to the agency to propose rules that further the statutory purposes. (3) As long as Congress makes this delegating guidance sufficiently clear, it is not delegating improperly. If Congress’s guidelines are too vague or undefined, it is (in essence) giving away its constitutional power to some other group, and this it cannot do.