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Publisher Summary This chapter summarizes the recent studies on the development of two distinct classes of efferent projection neurons in the mammalian cortex: the callosal and subcortically projecting. The cortex consists of six major layers which are distinguishable on the grounds of both cellular morphology and connectivity. The two major layer 5 efferent projection classes of the neocortex, the callosal and the subcortical, are distinct populations of projection neurons generated at the same time in development. The evidence suggests that the distinction between the two classes is determined early in development, either before or around the time that the neurons are generated. The projection class phenotype could be determined by cell-cell interactions such as those hypothesized to occur in the vertebrate retina. Alternatively, projection class could be determined by lineage dependent mechanisms. Lineage has been shown to be an important determinant of glial cell phenotype in cortex and is correlated with the differentiation of neurons into pyramidal and non-pyramidal morphologies. Only direct analysis of the role of lineage in determining cortical projection phenotype allows one to distinguish between these two mechanisms. These determination events are then played out during normal development as the two projection classes differentiate, elaborating their specific axonal and, later, dendritic phenotypes.