Assessment is an indispensable component in any learning institution, be it formal or informal. Without assessment it is hard to know the strengths and weaknesses of both teachers and students in teaching and learning. The ultimate aim of any institution of learning is progress and it cannot be attained unless there is assessment. However, not all forms of assessment can bring about progress. Improvement in the teaching-learning process for the benefit of both teachers and students is always at the centre of formative assessment (Black &Wiliam, 1998). I designed this study to develop an in-depth understanding into the use of formative assessment strategies in the teaching of mathematics. This study took place in a private primary school in Karachi, Pakistan. It was conducted in classes four and five. I used a qualitative design of research, in which classroom lesson observations, post observation conferences, interviews and review of documents served as tools of data collection. Research participants were mainly teachers and students. I also got supplementary information from the head teacher to have a holistic picture of the whole study with regard to assessment practices in the school. The findings revealed that there were a number formative assessment strategies used in the teaching of mathematics at the time of the study. The assessment strategies that were being used at the time of data collection were: questioning, marking and feedback, review of the previous work, and end-of topic tests. It was also found that there were facilitating and constraining factors that affected the use of formative assessment. I conclude with recommendations for the school administrators and teachers of the school in which the study was conducted. I also propose further studies in the same area of research community, with special focus in developing countries to see how effectively formative assessment can be applied.