Changes in assessment practices in the direction of assessment for learning (AfL) can be a powerful force to enhance student learning. This article presents a case study of teacher learning in an AfL project in Hong Kong. In the Project, AfL strategies were adopted in Chinese Language and English Language classrooms at the junior secondary level. By examining subject leaders' and classroom teachers' experiences with the project reported in interviews, the paper enriches our understanding of the variations in the subject department contexts of teacher learning, the situatedness of teacher learning in terms of knowledge about student learning, understanding of AfL and curriculum leadership, as well as the relationship between teacher learning and changes in assessment practices. The article concludes with suggestions on fostering subject departments to be expansive environments for teacher professional learning and to be a vehicle of internal change, alerts the importance of integration of AfL into teachers' pedagogical content knowledge and the subject curriculum, and highlights the cultural dimension as an issue for further study in AfL. Copyright © 2010 Taylor & Francis Group, an informa business.