Piaget's theory has been influential in psychological discussions of development and cognition. His theory is commonly considered a structural theory which posits the existence of logical-mathematical structures that direct the way the developing subject comes to know the world. This paper will argue that such a characterization of Piagetian theory is misleading. Instead, it is suggested that for Piaget, the central problem was to explain the formation of cognitive structures, and Genetic Epistemology was his unique philosophic solution to this problem, which stresses the importance of the constructive "activity of the subject'. In light of this interpretation, the psychological implications of Piagetian theory are considered.