Formative assessment has been shown to have the potential to significantly enhance student achievement, but a strong research base about how to support teachers to implement such a practice is lacking. This is particularly so for a conceptualisation of formative assessment as a unity of integrated formative assessment strategies. The aim of this study is to investigate why the mathematics teachers who participated in a successful professional development programme in formative assessment developed their formative classroom practice to such an extent that it had a significant impact on student achievement. An analysis of data from teacher questionnaires and interviews shows that the teachers' actions can be explained by expectancy-value theory of achievement motivation. Characteristics of the professional development programme that the teachers experienced as important for their development of a formative classroom practice are identified, and the characteristics' affordances for the development of the teachers' expectancy and value beliefs are discussed.