Roughly 70 percent of worldwide energy consumption and a comparable share of global greenhouse gas emissions are attributable to cities, according to the World Bank.1 In recognition of their contribution and of the need to address global warming, more than 170 US cities have made commitments to reduce their greenhouse gasemissions.2 In urban areas, buildings represent 50 to 80 percent of energy consumption and greenhouse gas emissions, with variations in buildings’ contributions across major cities: 64 percent of emissions in Tokyo,3 66 percent in New York,4 72 percent in Boston.5 Cities that aim to reduce their carbon footprint must therefore engage with the building stock and its owners to achieve their climate goals. As cities, states, and countries design and implement programs to mitigate building-level emissions, this paper examines lessons from three decades of experience with flexible approaches to carbon policy and their applicability to building program design. The success of this new class of programs will depend on interactions with existing climate regulation.