By following a professional development project focusing on enhancing assessment competence amongst teachers, the current study examines how teachers use reflective writing and systematic discussions as tools for developing professional competence in assessment. More specifically, the paper aims at identifying conditions which influence and facilitate reflection over time. The study was a small scale qualitative study, and the analysis draws on nine teachers' written and oral reflections. The findings suggest that it is possible to identify three closely connected development phases; describing, explaining, and exploring the level of action. By studying the reflection processes throughout these three phases the paper illuminates various aspects of reflection as a tool in teachers' professional development.