The author examined the effects of professional development (PD) on literacy instruction using 75,689 lessons from 1,945 classrooms in 112 schools participating in the Study of Instructional Improvement. Stratifying over 94 pretreatment covariates, including prior instruction, the results revealed the importance of PD as a lever for changing teacher practice. Teachers receiving intense PD in comprehension offered 10% more comprehension instruction than teachers not receiving intense PD. Similarly, teachers receiving intense PD in writing offered 13% more writing instruction and had students write 12% more text than other teachers. Furthermore, these estimates may be lower bounds, because they were estimated over a single year and because there was demonstrated potential for additional influences of the school context. (Contains 11 tables and 13 notes.)