This book deals with cognitive development from the perspective of the author's theory of conceptual learning and development (CLD) and from a Piagetian perspective. Chapter I provides an overview of CLD theory and also identifies and discusses the Piagetian constructs of stage, groupement structures, and equilibration. Chapter II focuses on the sampling design and the tests used in three studies of cognitive development (Kessen 1960, Nesselroads & Reese 1973, and Wohlwill 1973) that were carried out simultaneously using data from the same four longitudinal groups of studies. To supplement the condensed version of CLD theory, given in Chapter I, the major descriptive principles of cognitive development are discussed in Chapter III. The factor structure based on CLD theory is presented in Chapter IV. Also presented in this chapter are the results performed on the CLD tests and on educational achievement tests in grade 6, and of CLD tests and selected Project Talent tests in grades 9 and 12. Results from three studies of the Paigetian concrete operations stage in middle childhood are presented in Chapter V. Predictions that are based on expected relationships not only between the CLD measures and both the Piagetian and the groupements measures, but also between and within the Piagetian and the groupement measures, are tested in Chapter VI. Chapter VII presents a synthesis of the preceding findings regarding the developmental progression of the CLD, Piagetian, and groupement items during the school year. Chapter VIII discusses the relative importance given to the developmental factors and to learning in the Piagetian and CLD approaches. The final chapter presents an analysis of the relationship between psychological theories in general and their possible application to educational practice. Data from the studies discussed in this book are summarized in tables and presented in the appendix. (Author/MP)