The behaviors that are clinically relevant to language evaluation include speech, verbal problem solving, comprehension, rhetoric, and other communication skills. These behaviors may serve as markers against which we may measure retardation, unusual advance, or deviance. Following a brief review of the various language behaviors that a clinician may observe to tap the hypothetical language acquisition device, the author suggests the various combinations of signs that should help to differentiate among general retardation, specific language lags, and the specific language deviance shown by children with pervasive developmental disorders and dysphasia.