In this Article, the authors survey how agencies create substantive regulations through traditional rulemaking, negotiated rulemaking and litigation. Using public choice analysis, the Article relates agency choice to the agency's incentive structure. The Article also shows how the different forms of regulatory activity influence the content of agency regulations. Using a case study of EPA's regulation of heavy-duty diesel engines, the Article examines EPA's choices over thirty years as a means of testing the proposed theory. Finally, the Article concludes with a critique of allowing agencies to choose how they will regulate because the choice allows agencies to evade constraints imposed by Congress and the President and so diminishes political accountability.