A prerequisite for AfL to be successfully implemented in the classroom is the teachers' assessment practice. In many contexts, including the Norwegian, AfL has not been successfully dealt with during initial teacher education, and there is a need for qualified teachers to engage in professional development in AfL. This article first discusses different perspectives of professional development, not all of which lead to change of attitudes and beliefs resulting in changed practice. Difficulties in implementing AfL beyond the individual teacher and reach the full teaching staff is presented through experiences from a Norwegian research and development project. The next project to be briefly discussed focuses on engaging teachers in developing their own criteria for good classroom assessment. Finally the paper proposes that a major obstacle to teachers' professional development in AfL is the competence of teacher educators in AfL. Do those who are to guide and support professional development in schools hold the essential knowledge and competence themselves? The conclusive argument is that for AfL to be common assessment practice in schools there is a need for development processes at various levels of the education system. (Contains 2 figures.)