This part introduction presents an overview of the key concepts discussed in the subsequent chapters. The part discusses social developments have an even closer affinity to language acquisition than do sensorimotor conceptions of the world. It presents an aspect of development that has been somewhat ignored by those psycholinguists whose psychological orientation has been cognitive rather than social. The part argues that the cognitive, communicative, and social prerequisites to language, at least all those that have so far been proposed, appear to be well developed by the end of the first year. It addresses the second year remains largely unmapped as far as syntax is concerned nothing much seems to be happening until its end when two-word speech typically begins. The part is concerned with the social and interactive aspects of early language acquisition and uses the study of language as a case history in a dialectical approach to science rather than as the main focus of their presentation.