Current empirical research about the effects of acceleration on high-ability learners' academic achievement and social-emotional development were synthesized using meta-analytic techniques. A total of 38 primary studies conducted between 1984 and 2008 were included. The results were broken down by developmental level (P-12 and postsecondary) and comparison group (whether the accelerants were compared with same-age, older, or mixed-age peers). The findings are consistent with the conclusions from previous meta-analytic studies, suggesting that acceleration had a positive impact on high-ability learners' academic achievement (g = 0.180, 95% CI = -0.072, 0.431, under a random-effects model). In addition, the social-emotional development effects appeared to be slightly positive (g = 0.076, 95% CI = -0.025, 0.176, under a random-effects model), although not as strong as for academic achievement. No strong evidence regarding the moderators of the effects was found. (Contains 4 tables.)