This report analyzes the Advanced Clean Fleets (ACF) Regulation, which was adopted by the California Air Resources Board (CARB) in April 2023. The regulation requires owners of trucking fleets to phase out their gasoline- and diesel-powered vehicles and replace them with zero emissions vehicles within the next two decades. The regulation affects fleet operations beginning in 2026 and requires that 100 percent of the operations to be zero emission by between 2035 and 2042, depending on the type of fleet and truck. The regulation is intended to help California meet its extremely ambitious goal of net-zero CO2 emissions by 2045.