This paper discusses four kinds of reasons for studying child language. The first of the four, biological reasons, includes the desire to understand our own species and its place among other living things in the universe. The common human faculty for communication, the variability in language building, and the similarity of human communication to that of other species' are explored. The second group of reasons is social and addresses two qualities that distinguish human from other species' social behavior: (1) the existence of cultural differences; and (2) the continual construction and reconstruction of social reality by communicative interaction. The third set of reasons for studying child language is cognitive and acknowledges that knowing and thinking are states or activities highly characteristic of the human species and are worth systematic investigation. The fourth group of reasons is interventionist and focuses on normal language development as a basis for education, remediation, or therapy. It is proposed that by studying the playful, structure-building plasticity of child language development, we can answer the biological, social, cognitive, and interventionist inquiry. (MSE)