Background Improving children's oral language skills is an important focus of educational research and practice; however, relatively few interventions have demonstrated impacts on these skills. This work makes a unique contribution to our understanding of the effects of language-focused interventions in pre-kindergarten settings by examining impacts on both lower- and higher-level language skills as well as overall language comprehension. Objective The goal is to assess the impacts of business-as-usual pre-kindergarten with implementation of two versions of an experimental curriculum supplement, Let's Know!, designed to enhance three component language skills (vocabulary, comprehension monitoring, and text-structure knowledge) and overall language comprehension in pre-kindergarteners. Methods Eleven pre-kindergarten teachers and 49 low socioeconomic-status students participated. Teachers were randomly assigned to either business-as-usual, Let's Know! Broad, or Let's Know! Deep, unless they participated in a previous pilot study, in which case they were randomly assigned to either Let's Know! Broad or Deep. The Broad version included five different lesson types, whereas the Deep version included three lesson types with additional practice. Children's gains were assessed proximally with measures of vocabulary, comprehension monitoring, and text-structure knowledge and distally with a measure of language comprehension. Results Children in both experimental versions significantly improved their vocabulary skills relative to children who received business-as-usual instruction. For comprehension monitoring, children who received the Deep and Broad versions improved their scores relative to BAU children for Units 1 and 3, respectively. Improvement in language comprehension was only found for children who received Let's Know! Deep compared with business-as-usual. Conclusion This study provides initial evidence that the Let's Know! curricula may serve to foster young children's vocabulary, comprehension monitoring, and language comprehension skills. (author abstract)