This meta-analysis examines the effects of numerical competency development in mathematics classrooms. An exhaustive search of the quantitative literature yielded 17 studies with 39 effect sizes. The overall mean effect size, d = 0.88 (p < 0.05), indicates that there is a large positive summary effect of numerical competency development on mathematics achievement. The large overall effect size is complementary to empirical benchmarks for the average annual gains in mathematics knowledge across K-12 grade levels. Results suggest that numeracy and number sense are more effective than math fact fluency with regard to developing numerical competence. Subsequently, the majority of numerical competency studies were conducted in the early elementary grades. The results also indicate that the effect sizes are reduced drastically after second grade. The article concludes with explicit implications for mathematics teachers and researchers to support the evolution of numerical competency instruction in U.S. schools.