The author discusses a line of research examining the acquisition, organization, and use of knowledge associated with conceptual change in which she is engaged at the University of Maryland. It builds on foundational research by scholars in science, mathematics, and reading education as well as in motor learning and physical education pedagogy, examining student learning and performance in complex environments such as those found in games and sport. The author and her colleagues are extending these ideas to examine children's conceptual learning associated with physical activity and fitness content. Their research has led to National Institutes of Health funding to design, implement, evaluate, and disseminate a conceptually oriented health-related fitness program for elementary children, titled "Be Active Kids!" The author describes the process of conceptual change using examples from motor learning and physical education research representative of this perspective, and offers a description of the "Be Active Kids!" curriculum and some preliminary findings from research on children's knowledge growth and conceptual change. (Contains 9 figures.)