Background/Context: Most reforms in elementary education rely on teacher learning and improved instruction to increase student learning. This study increases our understanding of which types of professional development effectively change teaching practice in ways that boost student achievement. Purpose/Objective/Research Question/Focus of Study: Our three-year longitudinal analysis answers two main research questions: (1) To what extent do teachers' topic coverage, emphasis on memorization and solving novel problems, and time spent on mathematics instruction, predict student mathematics achievement growth? (2) To what extent does teacher participation in content-focused professional development predict the aspects of instruction found in our first analysis to be related to increases in student mathematics achievement growth? Population/Participants/Subjects : This study uses data collected by the U.S. Department of Education for the Longitudinal Evaluation of School Change and Performance (LESCP) in 1997, 1998, and 1999. The LESCP drew its sample from 71 high-poverty schools in 18 school districts in 7 states. Our student-level analyses include 7,588 observations over three years of 4,803 students assigned to 457 teachers. Teacher-level analyses include the same 457 teachers in 71 schools over three years. Research Design: This is a quasi-experimental longitudinal study. To answer our first research question, we employ a 4-level cross-classified growth model using MLwiN software, with time points nested within students, students cross-classified by teachers over the three years of the study, and teachers and students nested within schools. To answer our second question, we employ a series of hierarchical linear models (HLM) to test the relationship between instruction and professional development. Conclusions/Recommendations: We found that (1) when teachers in third, fourth, and fifth grade focused more on advanced mathematics topics (defined as operations with fractions, distance problems, solving equations with one unknown, solving two equations with two unknowns, and statistics) and emphasized solving novel problems, student achievement grew more quickly; (2) when teachers focused more on basic topics (defined as measurement, rounding, multi-digit multiplication, and problem solving) and emphasized memorizing facts, student achievement grew more slowly; and (3) when teachers participated in professional development that focused on math content or instructional strategies in mathematics (in Year 1), they were more likely to teach in ways associated with student achievement growth. Specifically, they were more likely to teach advanced topics and emphasize solving novel problems. Effect sizes ranged from 1% to 15% of a standard deviation.