In 2005, California enacted Assembly Bill 1493, also known as the Pavley Bill which set out to regulate greenhouse gas emissions from vehicular sources. For the Pavley Bill to take effect, however, California needed a waiver of Clean Air Act preemption. Two years after California’s request, the EPA, in an unprecedented move, denied the waiver. California promptly filed suit in the Ninth Circuit Court of Appeals and the case remains pending. This paper argues the Clean Air Act, as Congress originally envisioned, represents the best model for environmental legislation - a form of "modified" or cooperative federalism. In denying California’s waiver request, the EPA renders this cooperative federalism model ineffective; the EPA’s decision intrudes on states’ rights and disregards environmental concerns. As such, the Ninth Circuit should give full weight to the system of cooperative federalism and environmental protection espoused in the plain language and congressional intent of the Clean Air.