This study, sponsored by the Institute of Education Sciences (IES) in the U.S. Department of Education, examines whether some early elementary school math curricula are more effective than others at improving student math achievement in disadvantaged schools. A small number of curricula, which are based on different approaches for developing student math skills, dominate elementary math instruction--7 curricula make up 91 percent of those used by K-2 educators, according to a 2008 survey (Resnick et al. 2010). The main questions addressed by the study are: (1) What are the relative effects of the study's four math curricula on math achievement of first- and second-graders in disadvantaged schools? and (2) Are the relative curriculum effects influenced by school and classroom characteristics? The study is addressing these questions by examining the relative effects of four diverse curricula selected through a competitive process, recruiting schools to participate in the study, having publishers provide schools and teachers with curriculum materials and training, assessing student math achievement, and evaluating the effects of the curricula (including materials, training, content, and pedagogy) on student achievement. Other data (including school records, teacher surveys, and classroom observations) also were collected to examine whether relative curriculum effects are influenced by context and to conduct mediational analyses. The study team collected all data necessary to evaluate the four curricula. To measure the achievement effects of the curricula, the study team tested students at the beginning and end of the school year using the math assessment developed for the Early Childhood Longitudinal Study (ECLS-K) (West et al. 2000). The ECLS-K assessment is a nationally normed test designed to measure achievement gains both within and across elementary grades. The first- and second-grade results are based on students who were tested in both the fall and spring in those respective grades. As math skills have grown in importance in the workplace, so has the debate over how best to improve students' math skills. This study is an important step toward resolving that debate because it shows that four widely-used curricula have differential effects on student math achievement. These differential effects, which were measured after just one year of curriculum implementation, suggest that districts should carefully choose their math curriculum. (Contains 1 figure and 4 footnotes.)