Educators and policymakers have long debated which textbooks and their associated instructional approaches have the greatest impact on student learning in key subjects. Mathematica’s large-scale federal study of the effectiveness of four early math programs brings new clarity to what works in the early grades. The study, the largest of its kind ever to use an experimental design to study a variety of math curricula, includes a total of 110 schools. This report is based on the first cohort of 39 schools that began study participation during the 2006-2007 school year. Researchers found that achievement was significantly higher in schools assigned to Math Expressions and Saxon Math than in schools assigned to Investigations in Number, Data, and Space and Scott Foresman-Addison Wesley Mathematics. In addition, the better performing programs led to higher achievement for several student subgroups, including students in schools with low math scores and students in schools with high poverty levels.