In April 2020, the EPA and the National Highway Traffic Safety Administration (NHTSA) jointly issued final rules rolling back the EPA’s greenhouse gas emissions standards and NHTSA’s corporate average fuel economy standards for cars and light trucks of model years 2021 through 2026. The agencies acknowledged that achievement of more stringent standards was technologically feasible, that their actions would cause increased pollution and global warming and increase consumer fuel costs, and that their actions would achieve no net societal benefits even under the agencies’ error-ridden cost-benefit analysis. Public Citizen and the Consumer Federation of America joined other environmental and consumer groups in a lawsuit challenging the rules. A coalition of states led by California, as well as industry groups that promote clean-car technologies, filed parallel challenges. At the same time, a conservative advocacy group, the Competitive Enterprise Institute, challenged the new rules as being too stringent. The organizations and states challenging the rules filed their opening brief on January 2021. When President Biden took office, he announced that he was directing the agencies to reconsider the Trump rollbacks, and the cases challenging the rollbacks were held in abeyance pending new actions by the agencies. In December 2021 and May 2022, EPA and NHTSA, respectively, issued new rules with substantially more stringent greenhouse gas emissions standards and fuel economy standards. The challenges to the Trump-era standards, however, remain in abeyance until court challenges to the Biden Administration standards are resolved in light of the theoretical possibility that the Trump standards might come back into effect.