Since 1765, five major curricular reform movements have catalyzed significant changes in North American medical education. This article describes each reform movement in terms of its underlying educational practices and principles, inherent instructional problems, and the innovations that were carried forward. When considering the motivating factors underlying these reform movements, a unifying theme gradually emerges: increasing interest in, attention to, and understanding of the knowledge-base structures and cognitive processes that characterize and distinguish medical experts and novices. Concurrent with this emerging theme is a growing realization that medical educators must call upon and utilize the literature, research methods, and theoretical perspectives of cognitive science if future curricular reform efforts are to move forward efficiently and effectively. The authors hope that the discussion and perspective offered herein will broaden, stimulate, and challenge educators as they strive to create the reform movements that will define 21st-century medical education.