For fifty years Bärbel Inhelder (1913-1997) was the research companion of Jean Piaget. In this unique volume, published in her honour, leading international researchers examine the various aspects of her work and ideas and her contribution to developmental psychology. Following an initial chapter establishing Inhelder's stature as an independent researcher in her own right, the various research topics that she explored are reviewed and discussed with specific reference to her own perspective and in the chronological order in which she approached them. While the book explores Inhelder's work with her more famous colleague, it also highlights areas of research in which her ideas were at variance with those of Piaget, such as mental imagery, and areas in which her innovations have not been fully recognised, such as her discovery of the formal operations stage - an event usually attributed to Piaget - and her introduction of longitudinal studies in the field of cognitive development. Her research, viewpoint and contribution in other fields such as mental retardation, learning, and cross-cultural issues in development are also discussed. The final chapter, written by Inhelder herself, deals with experimental reasoning in children and adolescents and provides a glimpse of her creativity.