In this study, I examined the relationship between teachers' mathematical knowledge and instruction. Twenty-one K-8 teachers who were enrolled in a master's program were followed for 3 years to study how their mathematical knowledge and teaching changed over time. The results of multilevel growth models indicated that gains in teachers' mathematical knowledge predicted changes in the quality of their lesson design, their mathematical agenda, and the classroom climate. Analyses of interviews and classroom observation data conducted with a subgroup of teachers revealed that in addition to the gains teachers made in their mathematical knowledge, their exit level of knowledge played a significant role in the quality of the changes in their practices.