This proposal focuses on the communicative competence and performance among persons with mental retardation of widely varying ages and levels of development. The project should provide data that can contribute to an integration of psycholinguistic, cognitive, and behavioral theory about language, as well as adding to the knowledge of the ways in which humans use (and can be taught) language in social context. The plan is to use our developing research methodology to clarify the developmental history of several crucial communication issues: 1. Categorization, including individual differences in acquisition, performance, and retention. 2. Progression from natural gestures to symbolization, and from nonverbal concepts to language. 3. Verbal mediation of problem solving. 4. Generalization and transfer, including the facilitation of decontextualization. Issue one, categorization, is integrated into each of the six projects as a unifying focus for the research program. Issues two, three and four are also crucial in the acquisition of communication competencies. The last three issues are domains of communicative development into which categories are mapped. Within these organizing issues, we use methods that stem from behavior-analytic, cognitive, and psycholinguistic theory.