This meta-analysis examines the effects of vocabulary interventions on pre-K and kindergarten children's oral language development. The authors quantitatively reviewed 67 studies and 216 effect sizes to better understand the impact of training on word learning. Results indicated an overall effect size of 0.88, demonstrating, on average, a gain of nearly one standard deviation on vocabulary measures. Moderator analyses reported greater effects for trained adults in providing the treatment, combined pedagogical strategies that included explicit and implicit instruction, and author-created measures compared to standardized measures. Middle- and upper-income at-risk children were significantly more likely to benefit from vocabulary intervention than those students also at risk and poor. These results indicate that although they might improve oral language skills, vocabulary interventions are not sufficiently powerful to close the gap--even in the preschool and kindergarten years. (Contains 8 tables, 1 figure, and 1 note.)