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 competence in assessment. More specifically, the article aims at identifying conditions that influence and facilitate reflection over time. The study is a small-scale qualitative study from a Norwegian context, and the analysis draws on nine teachers' written and oral reflections. The findings suggest that it is possible to identify three closely connected developmental phases in these reflections; describing, explaining and exploring the level of action. By studying the reflection processes throughout these three phases, this paper illuminates various aspects of reflection as a tool in teachers' professional development.