Subject knowledge is widely viewed as important for teaching, and professional development (PD) often aims to build such knowledge. This brief synthesizes findings from three large-scale random assignment studies of PD that were conducted by the National Center for Education Evaluation and Regional Assistance in the Institute of Education Sciences. Although the PD programs in each study were different, they all emphasized building teachers' content knowledge or knowledge about content-specific pedagogy. The programs combined summer institutes with periodic teacher meetings and coaching during the school year. These programs were compared to the substantially less intensive PD that teachers typically received in study districts. The studies found that the PD boosted teachers' subject knowledge and some aspects of instructional quality, but did not have a positive impact on student achievement. The studies also found that most of the measured aspects of teachers' knowledge and practice were not correlated with student achievement. This consistent pattern of findings suggests that future studies should seek to better understand on what aspects of teacher knowledge and practice PD should focus, and how PD can achieve a larger impact on this knowledge and practice.