This brief aims to help educators understand the implications of math curriculum choice in the early elementary grades by presenting new findings from a study that examined how four math curricula affect students' achievement across two years--from 1st through 2nd grades. The four curricula were (1) Investigations in Number, Data, and Space (Investigations); (2) Math Expressions; (3) Saxon Math (Saxon); and (4) Scott Foresman-Addison Wesley Mathematics (SFAW), which the developer revised and renamed enVision Math (enVision) during the study. These curricula are widely used and differ in their approaches to teaching and learning. Within districts, we randomly assigned one of the four curricula to each school that participated in the study. After one year (by the end of 1st grade), students taught with Math Expressions and Saxon made greater gains in achievement than students taught with Investigations and SFAW. After two years (by the end of 2nd grade), Investigations students continued to lag behind Math Expressions and Saxon students, while SFAW/enVision students caught up to Math Expressions and Saxon students. Therefore, Math Expressions, Saxon, and SFAW/enVision improved 1st-through-2nd-grade math achievement by similar amounts, and all three outperformed Investigations. Our findings also suggest that switching between some of the study's curricula does not harm student achievement and can even be beneficial. (Contains 24 endnotes, 3 figures, and 2 tables.) [For "After Two Years, Three Elementary Math Curricula Outperform a Fourth. NCEE Technical Appendix. NCEE 2013-4019", see ED544187.]