Researchers and policy makers increasingly recognize the need to adapt to future changes in climate, given that past emissions of greenhouse gases have already committed the world to some level of climate change. However, the current understanding of the costs and benefits of adaptation measures is still fairly rudimentary, and far from comprehensive. An assessment is presented of the current state of knowledge on the magnitude of adaptation costs in the United States. While incomplete, the studies suggest that adaptation cost could be as high as tens or hundreds of billions of dollars per year by the middle of this century. Key studies are identified in each sector, and the cost estimates and approaches to cost estimation are surveyed. Methodological issues are highlighted in interpreting, comparing, and aggregating adaptation cost estimates. Policy recommendations are made along with appropriate steps to make future adaptation cost studies more comparable within and across sectors and more accessible and relevant to policy and decision makers.