This study reports a meta-analysis that synthesizes the empirical research on the role of language aptitude in second language grammar acquisition. A total of 33 study reports were identified including 17 predictive studies that investigated the correlations between aptitude and ultimate L2 attainment and 16 interactional studies that examined the interface between aptitude and the effectiveness of instructional treatments. These studies generated 309 effect sizes and involved 3,106 L2 learners. It was found that aptitude showed an overall moderate association with L2 grammar learning, "r" = 0.31, 95% CI = 0.25-0.36. Subsequent moderator analysis demonstrated that high school students were more likely to draw on aptitude than university students and that aptitude was more strongly correlated with explicit treatments than implicit treatments. The role of aptitude was more evident for younger learners than older learners in predictive studies whereas the opposite was true in interactional studies. The results suggest that language aptitude as measured via traditional aptitude tests is a set of cognitive abilities that were more implicated in initial stages of L2 development and conscious learning conditions. The findings are valuable to resolving some long-standing controversies surrounding language aptitude.