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Originally published in 1976, the authors present a theory of cognitive development based upon an information-processing approach. This approach leads to the presentation of precise models of performance on a number of tasks derived from a set of critical quantitative concepts: elementary quantification, number concepts, conservation and transitivity. These models encompass both early and late developmental stages, and a process model of the developmental mechanism itself is outlined. Here is one of the first attempts to apply the information-processing view of cognitive psychology to developmental issues raised by empirical work in the Piagetian tradition. It includes an extensive analysis of the processing demands of several of the classic tasks and describes the development of a system capable of performing a wide range of other tasks, including the ability to be self-modifying. It provides an introduction to general concepts and detailed properties of cognitive models stated as production systems. It will be most valuable for students in cognitive development and related courses in developmental, cognitive, and educational psychology, as well as computer science.