In the absence of strong U.S. national climate change policy, California and New York, among other states, adopted relatively comprehensive and ambitious policies to cut greenhouse gas emissions during the 2000s. They adopted these policies despite political-institutional and other structural barriers similar to those found nationally, which shows that political actors have significant scope for taking effective action even under structural constraints. This article explains the adoption of climate policies in these two leading states by using a windows of opportunity approach, which analyzes how the convergence of problem and political events produces policy windows and hence opportunities for advocacy coalitions to mobilize successfully for the adoption of major policies. It uses this framework to explain four episodes of state-level climate policy adoption, including: motor vehicle emissions and renewable energy policy in California (2002); emissions reduction targets, a low-carbon fuel standard, and emissions trading in California (2006-2008); a renewable portfolio standard and emissions trading in New York (2002-2003); and an energy efficiency standard and stricter renewable portfolio standard in New York (2007-2010). The convergence of problem and political events, including scientific reports, focusing events, election results, political leadership, and interest-group mobilization, led to these bursts of innovation, which helped to overcome structural barriers to climate policy adoption.