Children often mispronounce words when learning their first language. Is it because they cannot perceive the differences that adults make or is it because they can't produce the sounds involved? Neither hypothesis is sufficient on its own to explain the facts. On the basis of detailed analyses of his son's and grandson's development, Neil Smith explains the everyday miracle of one aspect of first language acquisition. Mispronunciations are now attributed to performance rather than to competence, and he argues at length that children's productions are not mentally represented. The study also highlights the constructs of current linguistic theory, arguing for distinctive features and the notion 'onset' and against some of the claims of Optimality Theory and Usage-based accounts. Smith provides an important and engaging update to his previous work, The Acquisition of Phonology, building on ideas previously developed and drawing new conclusions with the aid of fresh data.