In this case study, we follow the career of one student teacher (Ellen) from her pre-service training to her first year as a mathematics teacher. As part of a research project about what it means to become a mathematics teacher, the aim of the study was to gain insight into the development of formative assessment competence. To accomplish this, we used data from video-recorded lessons and audio-recorded conversations to depict changes in classroom practice and reflections on that practice during the study. Our findings show that Ellen made incremental changes in her classroom practice and that these had an impact on classroom interaction, formative assessment processes, and student involvement in those processes. Corresponding changes were identified in Ellen's reflections, which also indicated that she increased her insight into formative assessment. We discuss our findings in relation both to what it means to become a mathematics teacher and to the development of formative assessment competence.