Formative assessment is a key competence of professional teachers, yet complex and demanding to develop. Thus, beginning mathematics teachers (BMTs) cannot be expected to have fully developed this competence. In an ongoing study we examine and depict similarities and differences between four BMTs' use of formative assessment in practice. We will use data from video-recorded observations to analyze to what extent and in what ways the BMTs use five key strategies in formative assessment practice and the main idea of this practice, that is, to use information about students' learning to adjust teaching and learning to the students' needs. The study will provide insight into crucial aspects of formative assessment that BMTs need to learn about and reflect on during and after teacher education. Implications for teacher education are discussed.