Under the Clean Air Act, California is permitted to set its own vehicle emissions standards so long as it receives a waiver from the Environmental Protection Agency. California has done so regularly since the 1960s. However, it is unclear if the Trump Administration will continue to grant future waivers or review prior ones. This issue brief provides a background of the Clean Air Act vehicle emissions waiver and the recent California standards to which it has applied, including a waiver application denied under President Bush when California first attempted to establish vehicle emissions standards for greenhouse gases. It then describes recent developments in federal vehicle emissions standards and explores possible implications for California’s program.