This article reports on the teacher development that occurred during a two-year research project on the formative assessment practices of primary and secondary school teachers of science. The teacher development involved the teachers' professional, personal and social development. The project focused on clarifying what it was that served as formative assessment in the classroom. Personal development focused upon appreciating the risks and uncertainties involved in responding to what students are learning and acknowledging the importance of teacher confidence. Social development involved a re-examination of the nature of student-teacher interactions and of the complex and often contradicting roles of the teacher in relation to teaching, learning and assessment.