The adult neocortex, a distinct region of the mammalian cerebral cortex, is characterized by numerous anatomically and functionally distinct areas. Many of the connectional and architectural features that distinguish areas in the adult neocortex are not evident in the immature neocortex. A central issue in understanding neocortical area differentiation is determining the relative contributions of genetic and epigenetic factors in the emergence of area-specific features during neocortical development. A model system for this issue has been the rodent somatosensory cortex, which uniquely contains "barrels," anatomically evident functional groupings of cortical neurons and thalamocortical afferents that, in the tangential plane of cortex, are arranged in a pattern that reflects the distribution of vibrissae on the rodent body surface. Here, we address the role of thalamocortical afferents in the differentiation of barrels and their patterning in the context of discussing the specification of neocortical areas.