This study reports on a large-scale implementation of Assessment for Learning (AfL) in a Swedish municipality. The implementation was founded on two principles: (1) teaching should be informed by educational research; (2) to be successful teachers' professional development needs to be based in everyday classroom practice. From these principles, AfL was chosen as a strand of educational research to inform teaching and 'Teacher Learning Communities' were chosen as a vehicle for professional development and for implementing AfL practices. Findings indicate that the project has been successful in bringing about a change in how teachers talk about teaching and learning and in changing teachers' pedagogical practice towards AfL. Findings also suggest that AfL practices are mostly teacher-centred, which means that the teachers still take most of the responsibility for the assessment. This leads to high workload for the teachers and may also hinder students from taking responsibility for their learning.