Existing literature on the impact of Common Core State Standards in Math has shown little benefit, but has not examined variation in outcomes based on implementation strategies, student subgroups, or outcomes other than test scores. We use a difference-in-differences approach with school fixed effects to compare outcomes in pre- and post-standards years across schools with different levels of participation in professional learning around the standards in the middle grades in Chicago. Post-implementation, there were significantly greater improvements in standards-aligned instructional practices, math grades, pass rates and test scores in schools with more extensive professional learning around the standards, especially for students with low initial achievement. We discuss why Chicago might have seen positive results, including the district emphasis on professional learning around the practice standards and its teacher leader model.