The general objective of our research is to study the human capacity for language. We aim to investigate to what extent the overall form and organization of language is determined by the articulatory and perceptual modality in which it has developed, and to what extent they represent more fundamental aspects of human cognition. As a research tool we study American Sign Language, the system of hand signs developed by deaf people in the absence of speech. We find that ASL differs dramatically from English and other spoken languages in some of the mechanisms by which its lexical units are modified. For the form of its inflectional and derivational processes the mode in which the language develops makes a crucial difference. We propose to study the acquisition of three contrasting subsystems of American Sign Language: 1) the system of morphological processes which operate on signs, simultaneously changing or adding specifications for dimensions of movement to the root form of the sign, 2) compounding processes which are essentially sequential combinations of signs, bound together by particular rhythmic properties which differentiate them from phrases; 3) referential indexing, a system which includes the establishment of loci for non-present referents in the signing space between signer and addressee, and anaphoric reference by pointing or by inflection of the verb. Detailed studies of spontaneous mother/child conversations on videotape will be supplemented with intervention techniques, including eliciting procedures, tests of morphological processes, comprehension tests, systematically increasing the opportunities for reference to complex domains of quantification, temporal recurrence and focus.