US climate policy is in a state of significant uncertainty. Over the past year, federal policy has lurched backward through a series of regulatory rollbacks aimed at easing emissions limits on power plants, vehicles, and industry. In spite of this shift at the national level, policy in California—the world’s fifth largest economy—has grown more ambitious. The state’s cap-and-trade program now extends across all sectors of the economy. Its renewable portfolio standard was recently strengthened to target 100 percent clean electricity by 2045. And a series of technology standards now cover everything from electric vehicle sales to rooftop solar panels. What have been the major successes and challenges for California’s climate policy? What lessons does it offer for the nation and world? And how will the state move forward at a time of significant uncertainty at the federal level?