This paper and its analyses were commissioned in order to support a paper written by Barry G. Rabe for the Academy of Arts and Sciences Project on Durability and Adaptability in Energy Policy entitled “Leveraged Federalism and the Clean Air Act: The Case of Vehicle Emissions Control” (A revised version of this paper will appear as a chapter in a forthcoming Cambridge University Press book about the durability of the Clean Air Act that is being edited by Ann Carlson and Dallas Burtraw). First, we completed a literature review in order to understand why, historically, California has tended to lead in light-duty vehicle (LDV) regulations, while the federal government has tended to lead in heavy-duty vehicle (HDV) regulations. Then, we undertook an analysis to find any significant patterns, trends, or similarities among the waiver requests so as to better understand the process. The analysis was split into two parts – one on light-duty vehicle (LDV) regulation requests, and one on heavy-duty vehicle (HDV) regulation requests – because of the contradictions in leadership mentioned above. For both analyses, we reviewed the text of the waiver request and identified the waiver by the category of regulations it sought to pursue (LDV or HDV), the type of request it was, and the type of public commentary it noted. We found that since 1968, 94 waiver requests were submitted regarding LDV provisions and 39 were submitted regarding HDV provisions. Only nine have ever been rejected and all of the rejected requests regarded LDV regulation. Furthermore, 68 of the 94 LDV requests were concerned with substantive matters, while in the HDV analysis, 26 of the 39 were. In addition, 33 of the 94 LDV requests acknowledged some sort of public commentary, while 16 of the 39 HDV requests did. Lessons from this federal-state relationship could be adopted and applied to other environmental policy issues such as water pollution or greenhouse gas emissions standards from industrial complexes.