In the absence of federal action, states and cities in the United States have taken a leadership role in addressing the climate crisis, attempting to drive carbon emissions reductions. In this chapter, we review subnational policy actions across four areas: states pricing carbon; state-level low-carbon electricity policies; state-level low-carbon transportation policies; and city-level climate mitigation policies. After reviewing the major policies in each of these areas, we also discuss the weaknesses in current state and city approaches to climate change mitigation. We argue that while these subnational actions are laudable, they are insufficient. In fact, many states are failing to act at all. Interest group opponents and partisan polarization are both impeding progress at the subnational level. We conclude with a series of open research questions on state and local climate action in the United States.