The objectives of this book are essentially threefold. Firstly, the book describes the birth of cognitive science. Secondly, it outlines the method of enquiry which characterizes cognitive science and through which it is defined. The method employed is constructivistic, and is carried out by means of the techniques of artificial intelligence, relying on the assumption that mental activity is inprinciple reproducible by a computer program. Thirdly, the book describes the state of the art in the relevant domains, with particular attention to application fields like pedagogics, human/machine interaction and psychotherapy. The developmental approach is emphasized, and each chapter highlights the fact that developmental aspects are essential to comprehend the steady mode of functioning that is achieved once a person has reached total maturity. The technical sections of the book are introduced without presuming that the reader has a greater understanding of the subject than that of a high school student. This means that the book is suitable both for experts for its methodological `arts, and also for novices as a coherent critical introduction to the field.