In the era of education reform in Hong Kong, on various government documents, different concepts and functions of assessment were introduced to the community. Formative assessment is stressed as a way to improve teaching and enhance the learning of pupils. Formative assessment takes place in classrooms. It provides short term feedback to teachers so that learners' strengths and weaknesses in relation to their progression can be identified. Then teachers can use the feedback in planning what to do next in order to facilitate pupils' learning as well as promote teaching effectiveness. In response to the education reform, the researcher, as a teacher educator, wanted to study how formative assessment can be implemented in local General Studies classrooms. Fifteen student teachers were invited to be co-researchers of the present study. During their student teaching, they implemented formative assessment, collected data on their teaching and reflected on the implementation. The lessons videotaped by each of the co-researchers show the extent of the implementation of formative assessment in the classrooms. There were variations in terms of rich questioning, effective feedback, self- and peer assessment, as well as regulation of activity. Some of the assessment practices were either diluted or corrupted, though most of the co-researchers proclaimed that they successfully implemented formative assessment in their student teaching. Based on the findings of the study, the researcher attempted to make suggestions for the practising teachers so that they may initiate, implement and sustain this educational change in their subject classrooms.