In this report, we describe the results of a rigorous two-year study of the impacts of a mathematics initiative called Ongoing Assessment Project (OGAP) on teacher and student learning in grades 3-5 in two Philadelphia area school districts. OGAP is a mathematics program which combines teacher formative assessment practices with knowledge of student developmental progressions to build deeper student understanding of mathematics content. OGAP includes teacher professional development, classroom resources, school-based routines for regular practice, and ongoing school-based supports. The study was conducted in 61 schools during the 2014-15 and 2015-16 school years, with OGAP randomly assigned to 31 schools and the remaining 30 serving as comparison sites. The results of this study showed that OGAP produced meaningful impacts on both teacher knowledge and student learning.