This study explores how teachers implemented an assessment reform in South Korea, with an analysis of different aspects of the reform. Using a mixed method design, this study reveals that the relation between policy and practice depends upon the nature of the changes that reform policies propose. Teachers' implementation varies in terms of different aspects of the reform. Teachers struggle with implementing its fundamental aspects. This study suggests that it is important to take into account variation in implementation as the developmental progress of the reform and attend to scaffolding to make teachers further progress toward fundamental changes in practice.