There is a widespread view in the research and policy communities that the quality of mathematics and science instruction offered to students in the United States is low. The widely discussed results of the Trends in International Mathematics and Science Study (TIMSS) suggested that the reasons for American students’ poor performance in mathematics and science are complex, but at least partly due to weaknesses in the knowledge and skills of those teaching the subjects. In order to enhance teaching in these content areas, states and school districts need to act on what research has discovered about professional development (PD) and instructional improvement. The purpose of this brief is to share results from a major PD effort that extend our understanding of what is entailed in creating improvements at scale.