In many parts of the world, there is an increased demand for effectiveness and equity in education and for the development of students who see themselves as lifelong learners to meet economic and social challenges. Assessment for learning (AfL) can play a significant role in achieving these three demands. The purpose of AfL is to inform learning, not to measure it or sum it up; it is assessment that focuses on learning as it is taking place, not at the end of a sequence of learning; and it is intended to move learning forward from its current status. There is ample evidence that when effectively implemented AfL can improve learning for all students. However, less attention has been paid to the impact of AfL on the development of students' mathematical identities and to supporting equity goals. This paper uses a practice-based example to illustrate the benefits of AfL to student learning, to students' identities as doers of mathematics and to equity in the classroom. Consideration is also given to the particular case of English learners to illustrate how effective AfL practices can also support both language and concept development.