To be sustainable into the future, cities must be prepared for the impacts of global climate change. As greater variability in temperature and precipitation emerge, urban areas will need to navigate a host of challenges ranging from drought and heat waves to more frequent storms and flooding. Although there is a critical need for cities to initiate action to protect their built, natural, and human environments from these and other impacts of climate change, adaptation is an emerging policy domain in which there are few resources and best practices available to support planning and implementation. Despite limited guidance and support from national governments and the international community, some cities are finding ways to innovate and initiate citywide adaptation programs The sociology of institutions offers theoretical explanations for the behavior of early and late adopters. However, this body of scholarship does not fully address what motivates early adopters or account for how cities take action in issue arenas in which there are few institutional mandates, incentives, and best practices. Therefore in this research, I will compare cities in Japan and the United States pursuing adaptation planning to understand their motivations for taking action and the processes they follow in developing and implementing adaptation plans. In addition, given the emphasis cities have come to place on climate science and the mitigation of greenhouse gas emissions, I will investigate the ways in which climate impact assessments shape decisions and actions and how ongoing efforts at mitigation inform, align, or dominate adaptation initiatives. To study these issues, I will conduct semi-structured interviews with local government representatives and others participating in the adaptation process in two cities in Japan and two in the United States. The interviews will focus on motivations for initiating planning, planning process being followed, types of assessments conducted and ways the resulting information is being used, and the extent to which adaptation is linked to ongoing city climate action and sustainability initiatives. The data collection protocols will be aligned with those I have used to study adaptation planning in developing countries. As a result, this study not only will provide insight into the similarities and differences in adaptation planning in Japan and the United States, but will form the basis for comparisons of climate action in developed and developing countries. The findings of this research will generate new scholarly insights into what drives local governments to innovate and demonstrate leadership in emerging policy domains. In addition, the findings will enhance our knowledge of how scientific assessments influence decision making. Further, by comparing initiatives in different countries, this study will advance our understanding of the ways in which local and national differences affect planning processes. This research also will offer lessons to policy makers on how adaptation planning can be initiated and sustained as well as provide information about the types of resources cities need in order to be prepared for the impacts of climate change.