Child development is today conceptualized as an essentially social process, based on incremental knowledge acquisition driven by cultural experience and social context. We have “social” brains. The importance of cultural processes were recognized by the theory of Vygotsky (1978, 1986), who also highlighted the key role of language for cognitive development. Vygotsky argued that cognitive development did not just happen in the brain of the individual child, but depended on interactions between the child and the cultural “tools” available for mediating knowledge. The main cultural “tool” (form of symbolic representation for knowledge translation, e.g., writing, pictures, maps) discussed by Vygotsky was language, which he conceptualized as a psychological “tool” for organizing one’s own cognitive behaviour (e.g., via “inner speech”). The perspective adopted in this review is that the inter-relatedness of social and cognitive processes in the child are fundamental. Cognitive development is argued to occur within three “foundational domains”, naïve physics (knowledge about the physical world of objects and events), naïve biology (conceptual knowledge about the world of animates, inanimates and artefacts) and naïve psychology (understanding and predicting people’s behaviour on the basis of psychological causation) (Wellman & Gelman, 1998).