This article provides a longitudinal examination of how changes in more than 200 middle-grades mathematics teachers' instructional practices related to their (a) mathematical knowledge for teaching (MKT) and (b) instructional vision. Results of this multilevel regression analysis suggest that MKT and instructional vision are related to instruction in different ways. On average, MKT scores were positively related to the current year's quality of instruction but not to growth, whereas instructional vision was positively related to growth in instructional quality. Additionally, the analyses revealed different patterns of change depending on teachers' instructional vision and practice at the outset of the study.