The Administrative Procedure Act is a 1946 law that sets forth the procedure under which Congress can establish agencies to administer the regulations of United States Federal Law. When plaintiffs want to challenge the actions of the government; the APA provides a cause of action for them to do so, when the act of Congress is arbitrary, capricious, or otherwise not in harmony with the law.
In Wong Yang Sung v. McGrath, 339 U.S. 33, 37-41 (1950), the U.S. Supreme Court explained the issues which caused the enactment of the Administrative Procedure Act.