Piaget's theory of concrete operations structures d'ensemble is discussed, and certain predictions derived from it investigated in a longitudinal study of 120 children aged between 5.0 and 11.10. The data of these testings were submitted to a variety of analyses. The results suggested that the development of concrete operations was unlike the succession of discontinuous generalised stages which Piaget has postulated. His model is criticised as taking insufficient account of individual differences and of the structure of the testing situation.