The historical development of research on functional heterogeneity of the prefrontal cortex is traced, and an outline is provided of the methodological concepts which have been the basis of such research. Current behavioral data indicate the presence of five subregions in the prefrontal cortex of the monkey. (a) Lesions of the principal sulcus caused impairment in only two tasks: spatial delayed responses and spatial delayed alternations. (b) The superior prefrontal convexity may have a kinesthetic function. (c) Animals with lesions of the inferior prefrontal convexity were impaired in a variety of problems, including delayed matching to sample, go/no-go differentiation, and delayed alternation. (d) Electrical stimulation of the medial orbital cortex triggered autonomic effects, and lesions produced emotional disturbances and increased responding in extinction of bar-pressing for food. (e) Cells in the arcuate concavity had large, visual receptive fields; electrical stimulation elicited ocular movements, and lesions caused amblyopia. Only after acknowledging the functional heterogeneity of the prefrontal cortex, may a process be sought which is shared by all the prefrontal subregions. © 1979 Academic Press, Inc.