This paper explores climate change impact assessment, an area of increasing importance in regard to climate change science and policy-making. Through an historical overview of impact assessment and through case studies of sea-level rise and health impact sectors we document some of the major trends and debates that have characterized the impacts field. Our findings reveal ways in which the definition and analysis of impacts reflect aggregation of scientific, political, and societal issues. We also suggest that impact assessments can be thought of as "trading zones" in which negotiations take place between many actors over data, research priorities, participation, and methodological issues. These negotiations, in turn, have important implications for knowledge and power. For example, impacts and assessments of impacts are closely tied to organization within the scientific community, dominance of various research methodologies, boundaries that differentiate science and policy, and the viability of certain climate change policy responses.