Originally published in 1973, this book is an account of how the child learns the sound system of his native language, or how he learns to speak. A theory of the acquisition of phonology is derived from a detailed and rigorous analysis of the developing speech of a young child observed over a period of two years. The details of this analysis are elaborated in depth in chapters two and three and the major results of the study are given in chapter four. The final chapter is devoted to the implications of language acquisition for linguistic theory in general and generative phonology in particular. In addition to the obvious relevance of this work to general linguists and psychologists working on language acquisition, it was of considerable importance to speech therapists and all those involved medically with the observation and treatment of infant speech, in that it provided a characterisation of normal development which could act as a yardstick by which to measure abnormal or pathological conditions.