This book sets forth a new model of development from a causal perspective. As this is an area vital to several disciplines. It has been written at multiple levels and for multiple audiences. It is based on the work of Piaget and Neo-Piagetians, but also covers other major models in development. It has elements that make it attractive as a teaching text, but it is especially research-focused. It has clinical applications. It presents many new ideas and models consistent with the existing literature, which is reviewed extensively. Students, researchers, and practitioners should find it useful. The models presented in the present work build on models introduced in prior publications (e.g., Young, 1990a, 1990b; 1997).