This is the first book length survey of and introduction to the study of the child's acquisition of phonology. It contrasts data-based interactionist, cognitive models of phonological development with earlier deductive behaviourist and structuralist accounts. Setting these models in current neurophysiological perspectives, it integrates the flourishing independent research areas of infant speech perception and vocal production. The book traces the nature and timing of prosodic and segmental development with due attention to evidence of individual differences and from cross-linguistic studies. It describes the emergence of first words and the first phonological system against the background of the child's social and cognitive development in the first eighteen months.