ABSTRACT—Psychologists have known for over 20 years that infants begin learning the speech-sound categories of their language during the first 12 months of life. This fact has dominated researchers’ thinking about how language acquisition begins, although the relevance of this learning to the child’s progress in language acquisition has never been clear. Recently, views of the role of infancy in language acquisition have begun to change, with a new focus on the development of the vocabulary. Infants’ learning of speech-sound categories and infants’ abilities to extract regularities in the speech stream allow learning of the auditory forms of many words. These word forms then become the foundation of the early vocabulary, support children’s learning of the language’s phonological system, and contribute to the discovery of grammar.