Little is known about the relationship between knowledge and instructional quality of novice primary school teachers during their first 3 years of teaching mathematics. This study utilizes the Classroom video analysis and the Mathematical quality of instruction to examine changes in beginning teachers' usable knowledge and instructional quality, respectively, and their relationship in each year. The analyses reveal positive associations between knowledge and instructional quality in the second and third year of teaching. Two case study teachers illustrate the tight link between knowledge and practice and how different school contexts provide contrasting opportunities to learn from practice, thus contributing to the different patterns of knowledge and instructional quality over time. The findings suggest that usable knowledge of student mathematical thinking at the beginning of the profession supports teacher improvement in both knowledge and practice over time if the vision of the school context is aligned with student-centered instruction.