Joseph P. McDonald reports findings from a study of nine poverty-impacted schools in New York City striving to more effectively use student performance data in teaching. The study was one of 13 studies of data use at the classroom level across the United States funded by the Spencer Foundation -- an effort intended to fill a serious gap in research evidence on this policy-championed innovation. The article illuminates components of data use systems most associated with success in boosting student learning. It concludes that this sensible but complicated innovation can likely not be implemented well without distributed on-site leadership that includes teachers.