Although there is a considerable emphasis on inquiry-based, active learning in standards documents, curriculum documents, and textbooks, there exists a great deal of debate regarding the effectiveness of specific curricular and instructional approaches, including kit-based instruction. This study examines the efficacy of science kits in improving content knowledge. The method used involved treatment and comparison groups composed of 2,299 elementary school students in third, fourth, and fifth grades from ten different schools. In all the pairings but one, there were statistical differences in favor of the treatment groups or no statistical differences, suggesting that science kits enhance students' content understandings. (Contains 4 tables.)