A dynamic systems model is proposed on the basis of a general developmental mechanism adopted from the theories of J. Raget and L. S. Vygotsky, more particularly a mechanism based on the concepts assimilation versus accommodation and actual development versus zone of proximal development. In the model, action and experience have a distributed effect on contents (skills, knowledge, rules, action patterns, etc.) ordered along an abstract developmental distance dimension. After a mathematical treatment of the model, an overview is given of empirical evidence on continuous and discontinuous change. The dynamic model is then applied to the classic Piagetian and the neo-Piagetian models, models of continuous and discontinuous domain-specific change, and to models of cognitive strategies, transitions, microdevelopment, and inter-and imraindividual variability.Nineteen hundred ninety-six marked the centenary of the birth of two eminent developmental psychologists: the Swiss Jean Piaget and the Russian Lev Semyonovitch Vygotsky. The first emphasized the child as the maker of his or her own development, and the second emphasized the role of teaching and guidance and the formative effects of culture and society. Recently, a number of attempts have been made to show that their theories are less incompatible and mutually exclusive than is often believed (Classman, 1994;Tudge&Winterhoff, 1993). In the end, the most important question concerns which substantial and lasting contributions, if any, both Piaget's and Vygotsky's works have made toward a better understanding of the nature and mechanisms of mental and behavioral development. Lourenco and Machado (1996) have recently emphasized that a considerable part of the criticism of Piaget does not address or hardly addresses the fundamental issues that his theory contributed and the issues that are still worth considering.The general aim of this article is to demonstrate that the wide variety of characteristic patterns of cognitive development is in fact the expression of the intrinsic dynamics of dynamic systems that operate on the basis of major developmental mechanisms that can already be found in Piaget's and Vygotsky's works. If this can be demonstrated, the dynamic systems model based on these mechanisms can be seen as a way of integrating the fundamental aspects of a wide variety of theories, that is, as a way to arrive at a new synthesis. This synthetic theory does not deny the multitude of differences that exist between their theories but tries to find a convergence point in an abstract dynamic of basic principles of change that underlie development.The structure of this article is as follows: First, I select and discuss a number of concepts from Piaget's and Vygotsky's theories that refer to basic developmental mechanisms; second,