The mammalian isocortex is an intriguing mix of structural consistency and functional diversity. Across mammals, isocortex configuration is conserved to a very high degree, with visual cortex posterior, somatosensory cortex medial, and the associated secondary areas proximal to these primary areas. The classic, six- layered internal cortical architecture is conserved. Yet, cortical areas are also structurally and functionally distinct and can be distinguished on the basis of architecture, biochemistry, physiology and connectivity. Across species, the differences between areas are remarkably consistent. Is there something special about each cortical area that makes it adept at handling its particular input, or is it just a piece of a calculating machine that is indifferent to the modality of its information and simply performs routine transformations? If there are true differences in the cortical mosaic, when in development do they arise, and what feature of gene expression, connectivity or function carries the information that diversifies cortical areas?