This up-to-date presentation of language use and communication skills in atypical populations addresses questions on the essence of language, how it is shaped by normal cognitive, perceptual and social constraints and how it can be rehabilitated when these constraints are abnormal. The author covers: the evolutionary continuity of language as seen in communication systems in other species; the structure, power and processing of signed languages compared with spoken language; effect on language learning of language-deprivation during childhood because of abuse, neglect or deafness; developmental abnormalities which have a selective effect on language; and communication processes with brain damage or psychosis.