This book presents a new theory of how children acquire language and discusses its implications for a wide range of topics. It explores the roles of innateness and experience in language acquisition, provides further evidence for the theory of Universal Grammar, and shows how linguistic development in children is a driving force behind language shifts and changes. Charles Yang surveys a wide range of errors in children's language and identifies overlooked patterns. He combines these with work in biological evolution in order to develop a model of language acquisition by which to understand the interaction between children's internal linguistic knowledge and their external linguistic experience. He then presents evidence from his own and others' research in the acquisition of syntax and morphology and data from historical language change to test its validity. The model is the first to make quantitative and cross-linguistic predictions about child language. It may also be deployed as a predictive model of language change which, when the evidence is available, could explain why grammars change in a particular direction at a particular time. Knowledge and Learning in Natural Language is a pioneering work at the centre of current concerns in linguistics and cognitive science. It will interest all those concerned to understand and explain language acquisition, Universal Grammar, and language change.