The 1984 Supreme Court decision in Chevron U.S.A., Inc v. NRDC, 467 U.S. 837 established rules for judicial review of agency interpretations of statutes. An agency's rulemaking authority is derived through statute. Therefore, when an agency rulemaking is challenged in court, an agency's interpretation of their statutory authority is central to determining the legitimacy of the regulation.
The Chevron decision created a two part test to determine regulatory authority. First, the court must determine whether Congress spoke directly to the question at issue. If so, then the court defers to the statute. If Congress did not address the issue in question in the statute itself, then the court must determine if the agency's response to the statute is based on a "permissible" interpretation of the statute. If so, then the court must defer to the agency.
The Chevron case applies only to agency interpretations made in notice-and-comment rulemakings. (See Mead Doctrine).