Editor’s abstract: Piaget makes a distinction between development and learning – development being a spontaneous process tied to embryogenesis, learning being provoked by external situations. He proceeds to discuss the concept of an operation as an interiorized action linked to other operations in a structure. Four stages of development are enumerated – sensori-motor, pre-operational, concrete operations, and formal operations. Factors explaining the development of one structure of operations from another are discussed – maturation, experience, social transmission, and equilibration. Equilibration is defended as the most fundamental factor. Commenting on the inadequacy of the stimulus-response approach to understanding learning, Piaget presents evidence negating the effectiveness of external reinforcement in hastening the development of operational structures. These operational structures can be learned only if one bases the learning on simpler, more elementary structures – only if there is a natural relationship and development of structures. The learning of these structures is held to follow the same basic laws as does their natural development, i.e., learning is subordinated to development. Piaget concludes that the fundamental relation involved in development and learning is assimilation, not association.