The neocortex is composed of areas that are functionally, anatomically and histochemically distinct. In comparison to most other mammals, humans have an expanded neocortex, with a pronounced increase in the number of cortical areas, particularly those associated with visual processing. This expansion of the cortical sheet and the number of cortical fields underlies many complex behaviors associated with human capabilities including perception, cognition, language and volitional motor responses. We consider data from comparative studies as well as from developmental studies to gain insight into the mechanisms involved in arealization, and discuss how these mechanisms may have been modified in different lineages over time to produce the remarkable degree of variability observed in the visual neocortex of mammals. Because any phenotype is a result of the complex interactions between genotypic influences and environmental factors, we also consider environmental, or epigenetic, contributions to the organization of visual cortex.