Helping principals improve their leadership practices is a common use of federal funds and one way to improve instruction and student achievement. This study sought to better understand the effectiveness of an intensive principal professional development program focused primarily on helping principals conduct structured observations of teachers' classroom instruction and provide targeted feedback based on those observations. The report found that the program did not change principal practices as intended or improve students' achievement. However, improvements in some of the targeted principal practices were positively associated with improved student achievement. Attention on how to change these practices might be a potential avenue for improving the program. [For the one page highlights to this study, "The Effects of a Principal Professional Development Program Focused on Instructional Leadership. Study Highlights. NCEE 2020-0002," see ED599490. For the appendices to this study, "The Effects of a Principal Professional Development Program Focused on Instructional Leadership: Appendices. NCEE 2020-0002," see ED599491.]