This book presents current theory and research on cognitive change. Chapter authors were invited to discuss cognitive change from the perspective of its three main aspects. Its object (what changes in the mind during development?), its nature (how does change occur?), and its causes (why does change occur, or, in other words, what are the factors, internal and external, that are responsible for cognitive change?). Obviously, these are both old and fundamental questions and all theories of development attempted to answer one or more of them. Piaget was the first to provide a full set of answers to all three of these questions. His answer to these questions can rather easily be summarized as follows: operational structures (what?) change through reflecting abstraction, which organizes the results of assimilation and accommodation (how?), because of maturation, cultural influences, and self-organization (why?) (Piaget 1970, 2001). The rate of change during the course from birth to maturity varies systematically, accelerating and slowing down at different phases, so that cognitive development appears to be stage-like. In a sense, this summary of Piaget's theory is also an accurate summary of the present volume. If this had been the whole story it would have been nice because our task of writing an introduction to the book would have finished here. Fortunately, for the field at least, this is not the whole story. We have come a long way since Piaget in our knowledge about all three aspects of change and therefore an introduction is indeed needed.