This article synthesized science instruction studies with preschool and kindergarten children to understand the magnitude of science instruction's impact on young children's vocabulary outcomes. A total of seven studies that met criteria for the synthesis and provided sufficient data for the calculation of effect size were included. Science instruction has been examined using science intervention and vocabulary intervention with a focus on science. The overall mean effect size was moderate (0.66), suggesting that a focus on science increased young children's vocabulary outcomes. Science instruction appears to promote young children's domain-specific words. Educational implications of these findings suggest that there is support for using science instruction to increase the vocabulary performance of young children. (author abstract)