Abstract: In carrying out their duties, federal administrative agencies must often interpret statutes and regulations that are not entirely clear. Sometimes an agency’s interpretation of an ambiguous legal text may not seem like the best or most natural interpretation of that text. Nonetheless, a staple of modern federal administrative law doctrine is the principle of judicial deference to administrative interpretations of both congressional statutes and agency regulations. The seminal case on judicial deference to reasonable agency statutory interpretations is, of course, Chevron U.S.A. Inc. v. Natural Resources Defense Council, Inc. In the context of administrative interpretations of the agency’s own regulations, the leading authority is the Supreme Court’s 1945 decision in Bowles v. Seminole Rock & Sand Co., which held that an agency’s construction of its own regulation should be given “controlling weight unless it is plainly erroneous or inconsistent with the regulation.” More recent Supreme Court cases—including Thomas Jefferson University v. Shalala and Auer v. Robbins—have reaffirmed the Seminole Rock principle of judicial deference to an agency’s reasonable construction of its own regulations.