Two second-order meta-analyses synthesized approximately 100 years of research on the effects of ability grouping and acceleration on K–12 students’ academic achievement. Outcomes of 13 ability grouping meta-analyses showed that students benefited from within-class grouping (0.19 ≤ g ≤ 0.30), cross-grade subject grouping (g = 0.26), and special grouping for the gifted (g = 0.37), but did not benefit from between-class grouping (0.04 ≤ g ≤0.06); the effects did not vary for high-, medium-, and low-ability students. Three acceleration meta-analyses showed that accelerated students significantly outperformed their nonaccelerated same-age peers (g = 0.70) but did not differ significantly from nonaccelerated older peers (g = 0.09). Three other meta-analyses that aggregated outcomes across specific forms of acceleration found that acceleration appeared to have a positive, moderate, and statistically significant impact on students’ academic achievement (g = 0.42).