A dynamic systems model is proposed on the basis of a general developmental mechanism adopted from the theories of J. Piaget 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 intraindividual variability.