Children who know fewer words in preschool typically continue to have lower levels of vocabulary knowledge in higher grades than their peers who know more words. In fact, this difference continues to be pronounced and even widens as these children reach higher grade levels. Interventions that support vocabulary development and reading comprehension in early childhood have the potential to improve student language development, narrowing this gap. "World of Words" is a supplementary curriculum used to help young children in prekindergarten develop vocabulary knowledge, concept knowledge, and content knowledge in science. The curriculum includes intentional conversations and shared book readings of texts focused on science topics. The What Works Clearinghouse (WWC) reviews existing research on educational interventions to identify evidence-based programs and practices. This WWC intervention report summarizes the available evidence on the effects of "World of Words" on student outcomes. [For the Intervention Brief, see ED630041.]