This article examines the effectiveness of four elementary school math curricula: (a) "Investigations in Number, Data, and Space"; (b) "Math Expressions"; (c) "Saxon Math"; and (d) "Scott Foresman-Addison Wesley Mathematics" ("SFAW"). These curricula are distinct from one another and represent many of the diverse approaches used to teach elementary school math in the United States. The results are based on 39 schools that were randomly assigned to use the various curricula at the first-grade level. The results show that average spring first-grade math achievement of "Math Expressions" and "Saxon" students was 0.30 SD higher than "Investigations" students and 0.24 SD higher than "SFAW" students. These effect sizes mean that an average-performing student's percentile rank in math would be 9 to 12 points higher if the school used "Math Expressions" or "Saxon," instead of "Investigations" or "SFAW." We also conducted correlational analyses that examine whether curriculum-group differences in math instructional time and content coverage account for the differences in curriculum effects and found that a portion of the relative effects is due to differences in content coverage. (Contains 1 figure, 11 tables, and 33 footnotes.)