A discussion of functional language proficiency considers several widely-accepted theories of cognition, draws parallels between specific stages of intellectual maturity and development of second language proficiency, and suggests ways in which teachers may be able to help students progress from one stage to another within the context of foreign language instruction. The work of Lev Vygotsky linking linguistic and cognitive development is first outlined, and William G. Perry, Jr.'s study of adolescent and young adult development is examined. Piagetian theory concerning child and adolescent development in the context of education is also summarized, with descriptions of the four broad periods of intellectual development. Subsequent work by Bruner and others on the development of language together with cognition is reviewed. Suggestions for classroom application of these concepts focus on provision of a second language learning context as similar as possible to that of native language learning, with maximum exposure without immediate demands for production. The teacher should act as an interested and sympathetic interlocutor, not placing too many demands on the student. Emphasis on message and form must be balanced. A 17-item bibliography is included. (MSE)