How do children learn and how are new modes of thought developed? These questions have for years been of paramount interest to psychologists and others concerned with the cognitive development of the child. In this major work, originally published in 1974 and reporting on over ten years' research of the Geneva School, the authors carried the pioneering investigations of Jean Piaget to a new and remarkable level. As Piaget said in his foreword to the book: 'The novelty of the findings, the clarity of the theoretical interpretation, and the sometimes even excessive caution of the conclusions enable the reader to separate clearly the experimental results from the authors' theoretical tenets.' The authors' learning experiments with children were designed to examine the processes that lead to the acquisition of certain key concepts, such as conservation of matter and length. Detailed study of the progress of each individual subject revealed a number of features characteristic of situations that create conflicts in the child's mind and certain regularities in the way these conflicts are resolved. Such data threw new light on the dynamics of the development of cognitive structures as well as on basic mechanisms of learning at the time.