Randomized field trials were used to examine the impact of the Teacher Study Group, a professional development model, on first grade teachers' reading comprehension and vocabulary instruction, their knowledge of these areas, and on the corollary comprehension and vocabulary achievement of their students. The multi-site study was conducted during the 2004-2005 and 2005-2006 school years in three large urban school districts from three states: California, Pennsylvania, and Virginia. 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 scored 0.86 standard deviations higher on the comprehension measure and 0.58 standard deviations higher on the vocabulary measure than control teachers. TSG teachers also significantly outperformed control teachers on the teacher knowledge measure of vocabulary instruction (ES = 0.73). Confirmatory analysis of student outcomes focused on passage comprehension, reading vocabulary, and oral vocabulary; of these only the latter was marginally significant (0.44). Implications for teacher practice and future research are discussed.