One of the most controversial aspects of administrative law in recent years concerns agency decisions not to enforce the law. Such nonenforcement is often beneficial or, in any event, inevitable. A particular violation may be so distant from what Congress or the agency had in mind when the general prohibition was put on the books that enforcement makes little sense. Likewise, because agencies have finite resources, they cannot enforce the law in all situations. At the same time, however, nonenforcement can also raise difficult questions about basic notions of fairness and administrative regularity, as well as separation of powers concerns. Nonenforcement decisions can be particularly significant, moreover, because they often are not subject...