This theoretical paper discusses (1) the meaning of the concept of structure, how the concept of structure is used by psychologists, and how the concept can be defined, and (2) the role of the concept "structure" in cognitive development with specific reference to Piaget's vision of intellectual development. The meanings of the term structure in structural psychology, in gestalt psychology and in the structuralist movement are described, and it is emphasized that the concept of structure is a purely logical notion. Adjectives (like grammatical) serve only to modify rather than define the generic concept. Treating structure in this way implies that structural isomorphism has useful implications for work in new areas. Such analytical uses of structure are contrasted with Piaget's conception of cognitive structure which, it is alleged, has been used as grounds for his stages. The possible fallacy of assuming a relation between cognitive structures and cognitive stages in Piagetian theory is explained. Predictions concerning timing and sequence of stages based on this assumption are contrasted with Piaget's findings. (GO)