The goal of this study is to examine the trends, policies and issues of impact evaluation, and to draw policy implications for the introduction of impact evaluation in Korea's ODA evaluation system. To achieve this goal, this study first compares and analyzes the policy, evaluation system and various cases of impact evaluation from other donor agencies such as the World Bank, ADB, USAID, MCC, and JICA. Second, we analyze the major issues related to impact evaluation in details. Three issues are raised and analyzed: evaluability assessment, methodology design, and feedback of the result. Third, we conduct an impact evaluation using clustered-RCT to assess the performance of a health project supported by the Korean government in Vietnam. Finally, we analyze the tasks for introducing impact evaluation into Korea's ODA evaluation system at the level of an ODA-integrated evaluation system and implementing agency. This study also proposes a mid- to long-term roadmap to reform Korea’s ODA evaluation system.