In "Semantic and Conceptual Development," Frank Keil presents the first psychological investigation of the developing child's ontological knowledge. Building on previous philosophical work, Keil shows that ontological categories develop in a highly predictable progression. Moreover, Keil demonstrates that ontological development obeys a strong formal constraint on the relations among categories. Although there are many possible ontological systems, children appear to be inherently targeted to consider a system of only one sort. Keil's results represent exactly the sort of interdisciplinary study of the human mind which is gradually emerging as the new field of cognitive science. We are proud to publish his work as the first book in the Cognitive Science Series, which is designed to foster major empirical and theoretical contributions to this new field.