Sound play is important to child language development in that it contributes to the phonetic substrate, it is a factor in phonological development, and it is something to be learned as part of the socially acceptable use of language. Sound play progresses in three stages: (1) babbling, in which a gradual acquisition of phonetic units is built up and then narrowed down to approximate adult speech sounds; (2) expressive sound play, where the primary focus is the sounds rather than the meaning, and includes exploratory behavior, sound practice, and "pure" sound play; and (3) language games, in which sound play becomes more elaborate and where conscious, explicit, and socially prescribed rules for sound play are developed. A cognitive model views the child as an active rather than a passive language learner. This active role is evidenced in the child's selectiveness in exploration of input, his creativity in production of segments and patterns not in the adult phonology, and his formation, testing, and revision of hypotheses about the phonological structure of the language. (PJM)