A meta-analysis of vocabulary interventions in grades pre-K to 12 was conducted with 37 studies to better understand the impact of vocabulary on comprehension. Vocabulary instruction was found to be effective at increasing students' ability to comprehend text with custom measures (d = 0.50), but was less effective for standardized measures (d = 0.10). When considering only custom measures, and controlling for method variables, students with reading difficulties (d = 1.23) benefited more than three times as much as students without reading problems (d = 0.39) on comprehension measures. Gains on vocabulary measures, however, were comparable across reading ability. In addition, the correlation of vocabulary and comprehension effects from studies reporting both outcomes was modest (r = 0.43). (Contains 12 tables and 4 figures.)