In recent years Piaget's researches and theories have been recognized by educators as having a considerable bearing upon their activities. Several books have been written, all of which have made Piaget's work more widely known. A number of these which are particularly valuable are listed further on, under the heading, 'Other Sources of Reading in Piaget's Psychology.' It seems to the present author that the breadth and depth of Piaget's studies is such that there is room for many derivative works, each of which can make a useful contribution to the wider understanding of the significance of the psychology. However, it will be of help to the reader if, at this stage, some general points are made about how this book is orientated, and what basic principles underlie its format and content.