This study examined (a) developmental stability and change in children's private speech during the preschool years, (b) across-task consistency in children's self-speech, and (c) across-setting relations between children's private speech in the laboratory and their behaviour at home and in the preschool classroom. A group of 32 normally developing three- and four-year-old children was observed twice (six month inter-observation interval) while engaging in the same individual problem-solving tasks. Measures of private speech were collected from transcribed videotapes. Naturalistic observations of children's behaviour in the preschool classroom were conducted, and teachers and parents reported on children's behaviour at home and school. Individual differences in preschool children's private speech use were generally stable across tasks and time and related to children's observed and reported behaviour at school and home. Children whose private speech was more partially internalized had fewer externalizing behaviour problems and better social skills as reported by parents and teachers. Children whose private speech was largely task-irrelevant engaged in less goal-directed behaviour in the classroom, expressed more negative affect in the classroom, and rated as having poorer social skills and more behaviour problems. Developmental change occurred during the preschool years in children's use and internalization of private speech during problem-solving in the form of a reduction over time in the total number of social speech utterances, a decrease in the average number of words per utterance, and an increase in the proportion of private speech that was partially internalized.