Formative assessment has been shown to have the power to positively affect student achievement, but professional-development initiatives for teachers to substantially develop their formative assessment practices have been frequently unsuccessful. In this study, a random selection of Year-7 teachers participated in a professional-development programme (PDP) in formative assessment. Classroom observations, questionnaires, and interviews were used to investigate the changes made in the teachers’ classroom practices after the PDP, and the conditions and barriers they perceived to have been important in their endeavours to develop a formative classroom practice. The results show how all teachers developed their practice, but to different degrees, and that a majority of the implemented activities focused on the teacher as the responsible agent for the formative assessment process. The teachers were satisfied with most of the conditions provided during the PDP, but experienced barriers for change from the teaching conditions at their schools after the PDP.