In recent decades, states have exhibited remarkable leadership in environmental policy. This leadership is threatened by federal ceiling preemption, which prevents states from adopting regulations that exceed federal standards. While environmental law scholars have argued that the rise in federal ceiling preemption will undermine environmental policy, these critics have failed to take the arguments in favor of preemption seriously. Specifically, they have not addressed the risk that states may adopt tough environmental regulations because they can externalize costs to other states, or that a single, large, pro-regulatory state like California could effectively dictate excessively stringent national standards. This Note presents a more principled case against federal ceiling preemption in environmental law and contends that the cost-externalization argument’s practical application is limited. It illustrates this primarily through an extended case study of California’s regulation of greenhouse gas emissions from motor vehicles. The Note argues that state regulations that provide manufacturers with sufficient flexibility to meet standards without disrupting economies of scale can largely avoid externalizing costs to out- of-state consumers. It further contends that states may have to consider the interests of out-of-state producers when issuing regulations because, among other reasons, compliance costs will be partly internalized by in-state consumers and shareholders. The Note concludes that the merits of the cost-externalization argument must be carefully weighed against the benefits of decentralized policymaking in order to yield optimal environmental policy.