Climate change has been a salient issue for the countries of Northeast Asia (the People's Republic of China, Japan, the Republic of Korea, and Mongolia) in terms of impacts, adaptation, and mitigation. Preliminary analyses suggest that the costs of coastal protection are likely to be the largest adaptation costs across sectors in the region. In infrastructure, Mongolia will probably have the largest adaptation costs in percentage terms. In the PRC, there is a large 'adaptation deficit' to extreme events (e.g., flooding and cyclones) in the infrastructure sector that should be addressed now. There is a large mitigation potential in Northeast Asia - at least 7 gigatons of carbon dioxide equivalent in 2030. Furthermore, much of the potential is at negative cost. That is, they are economically justified regardless of their emission benefits. Regional cooperation is important as it can help reduce the total costs of implementing mitigation in Northeast Asia.