Piaget helps us to see the developmental significance of a child's failures and successes in thought and action during everyday experience by breaking down each activity into its separate mental elements. We have to tried to draw the educational implications from the developmental facts thus revealed. In recent years teachers have had to learn a great deal about mental measurement as this has become an important feature in our educational structure. This has led to much emphasis on the quantitative assessment of intellectual ability, since in most intelligence tests the main concern is with the number of right responses. In his 'open-ended tests' Piaget seeks to find in a large number of situations what it is that we take for granted which the children have not grasped. To do this he examines the processes of thought and the degree of success and failure, which should be of much greater diagnostic value to the practising teacher. It also gives further support to those who believe in the need for an individual approach to each child's learning. For many years, people who have worked in child centred education have had philosophical theory and intuitive judgment to guide them, but have lacked scientific justification for what they were doing. Piaget's work is now providing scientific evidence from experiments, with concrete examples and demonstration from children's behaviour for what was previously a matter of opinion. We have chosen the examples to cover a wide age range partly to emphasise the genetic approach and partly to appeal to as wide an audience of teachers as possible. In addition we tried to choose pieces that held special promise of applicability in schools.