Although substantial research has established how to teach word reading, the research base for teaching skills related to language and reading comprehension is more limited. We report a multistate experiment of a supplemental, whole-class, language-focused curriculum delivered in prekindergarten and kindergarten classrooms that was designed to improve children's language comprehension and thereby support later reading comprehension. We randomly assigned 69 prekindergarten classrooms (n = 361 children) and 56 kindergarten classrooms (n = 328 children) to receive language-focused intervention or to a control condition. Children in intervention conditions experienced one of two instantiations of Let's Know! (Let's Know! Broad or Let's Know! Deep) as implemented by their classroom teachers. Both instantiations provide four 30-min lessons per week of targeted instruction on key lower- and higher-level language skills over 25 weeks; the instantiations differ in the amount of practice afforded to particular skills. We measured children's comprehension-related skills (target vocabulary, comprehension monitoring, understanding narrative text and story grammar, understanding expository text) via curriculum-aligned measures during the academic year and their vocabulary and language comprehension at pretest and posttest. Multilevel analyses showed similar effects for both instantiations, with Let's Know! positively impacting some of the immediate tests of curriculum-aligned skills (vocabulary, comprehension monitoring, understanding of expository text) and also the posttest vocabulary outcome, but not standardized language comprehension outcomes; impacts on curriculum-aligned skills did not mediate effects on language comprehension outcomes. Results have implications for the Let's Know! theory of change as well as continued research on supporting children's language and comprehension skills. [This article was written with the Language and Reading Research Consortium.]