This monograph presents a theory of conceptual development, the main construct of which is the "central conceptual structure". These structures are defined as networks of semantic nodes and relations which represent children's core knowledge and which can be applied to a full range of tasks in a given domain. Comprehensive cross-cultural testing of model structures and a six-year programme of instructional research show how the transformation of these central conceptual structures have a powerful influence on subsequent knowledge acquisition. The work should interest cognitive scientists, educational psychologists and child development specialists.