A study was conducted to determine criteria to measure successful language development and to determine what factors might be considered to be the determiners of this development. Subjects were 16 children, aged 3 years 3 months, selected on an intuitive basis from the 64 children in the older age group to represent the full range of development, from most advanced to most retarded. The data for comparison were of two kinds: samples of the children's spontaneous conversation recorded in their own homes on a time sampling basis over a complete day and subsequently transcribed and linguistically coded and scores on a test of comprehension. Measures considered were mean length of utterance, syntactic complexity, auxiliary verb system, semantic complexity, and pragmatic range. Teachers' assessment of children's language development was also considered. It was concluded that the factor here called "command of the language system" could be readily discerned in the speech data and that the range of pragmatic functions in the speech addressed to the child was the best predictor of the child's language development. Study findings also suggested that there may not be a close relationship between command of the language and effective communication. (Author/MS)