This is one of those rare books that promises to change the way that psychologists view the central problem of developmental psychology. . .Siegler provides a cogent and convincing argument that variability is a constant in thought at all levels and provides the key to cutting through to the problem of cognitive change. In addition to providing a wide range of examples showing the centrality of adaptive variability in children's thinking at all levels, Siegler describes a methodology for describing developmental change as it progresses. Few will be able to read it without considering how to apply this model and methods to their own domain of interest. This book will serve as a handbook for anyone who wants to take up the challenge of taking development seriously.