Randomized field trials were used to examine the impact of the Teacher Study Group (TSG), a professional development model, on first grade teachers’ reading comprehension and vocabulary instruction, their knowledge of these areas, and on the comprehension and vocabulary achievement of their students. The multi-site study was conducted in three large urban school districts from three states. A total of 81 first grade teachers and their 468 students from 19 Reading First schools formed the analytic sample in the study. Classrooms observations of teaching practice showed significant improvements in TSG schools. TSG teachers also significantly outperformed control teachers on the teacher knowledge measure of vocabulary instruction. Confirmatory analysis of student outcomes indicated marginally significant effects in oral vocabulary.