AbstractA theory of language is outlined which emphasises the interactive nature of linguistic communication, and it is hypothesised that such interaction forms the basis for the child's acquisition of control of the language system. It is further suggested that differences in quality and quantity of interaction, in which meaning is negotiated in the shared construction of social reality, may be largely responsible for the ‘linguistic disadvantage’ which is the putative cause of the educational under‐achievement of many children. The paper describes the way in which these questions are being investigated in a longitudinal study of the language development of a representative sample of children, based on regular observations of the children's spontaneous linguistic interaction at home and at s