Abstract: In the last quarter-century, the Supreme Court has legitimated agency authority to interpret regulatory legislation, above all in Chevron U.S.A., Inc v Natural Resources Defense Council, Inc, the most-cited case in modern public law. Chevron recognizes that the resolution of statutory ambiguities often requires judgments of policy; its call for judicial deference to reasonable interpretations was widely expected to have eliminated the role of policy judgments in judicial review of agency interpretations of law. But this expectation has not been realized. On the Supreme Court, conservative justices vote to validate agency decisions less often than liberal justices. Moreover, the most conservative members of the Supreme Court show significantly increased validation of agency interpretations after President Bush succeeded President Clinton, and the least conservative members of the Court show significantly decreased validation rates in the same period. In a similar vein, the most conservative members of the Court are less likely to validate liberal agency interpretations than conservative ones and the least conservative members of the Court show the opposite pattern. Similar patterns can be found on federal appellate courts. In lower court decisions involving the EPA and the NLRB from 1990 to 2004, Republican appointees demonstrated a greater willingness to invalidate liberal agency decisions and those of Democratic administrations. These differences are greatly amplified when Republican appointees sit with two Republican appointees and when Democratic appointees sit with two Democratic appointees.