Let's Know! is a language-focused curriculum supplement developed through the Institute of Education Sciences' Reading for Understanding initiative aimed at supporting prekindergarten through grade 3 students' listening and reading comprehension. The current study reports results concerning the impacts of 2 instantiations of Let's Know! on students' comprehension-related skills (comprehension monitoring; understanding narrative and expository text, as supported by inference making and knowledge of text structure; and vocabulary) as proximal measures of efficacy. Results from the first cohort of a large, field-based, randomized controlled trial (N = 766 students across grades) indicate large, consistent, and statistically significant effects on curriculum-aligned comprehension monitoring and vocabulary probes relative to control, minimal effects on understanding narrative and expository text probes relative to control, and few differences across the two instantiations. Findings are interpreted with respect to the promise of Let's Know! for achieving intended comprehension impacts, and limitations and future directions are discussed.