An earlier study showed that the changes in teachers' classroom practice, after participation in a professional development program in formative assessment, significantly improved student achievement in mathematics. The teachers in that study were a random selection of Year 4 teachers in a Swedish mid-sized municipality. In the present study, we analyse and describe the characteristics of these changes in classroom practice, which were based on a combination of various strategies for formative assessment. Data were collected through teacher interviews and classroom observations. The teachers implemented many new activities that strengthened a formative classroom practice based on identifying student learning needs and modifying the teaching and learning accordingly. The characteristics of the changes the teachers made reveal the complexity of this formative assessment practice and why such developments of practice are likely to require major changes in most teachers' practices. We also discuss how such changes in practice afford new learning opportunities.