Research continues to find large differences in student achievement gains across teachers' classrooms. The variability in teacher effectiveness raises the stakes on identifying effective teachers and teaching practices. This paper combines data from classroom observations of teaching practices and measures of teachers' ability to improve student achievement as one contribution to these questions. We find that observation measures of teaching effectiveness are substantively related to student achievement growth and that some observed teaching practices predict achievement more than other practices. Our results provide information for both individual teacher development efforts, and the design of teacher evaluation systems. (Contains 15 footnotes and 7 tables.)