Corticostriatal projection neurons (CStrPN) project from the neocortex to ipsilateral and contralateral striata to control and coordinate motor programs and movement. They are clinically important as the predominant cortical population that degenerates in Huntington's disease and corticobasal ganglionic degeneration, and their injury contributes to multiple forms of cerebral palsy. Together with their well-studied functions in motor control, these clinical connections make them a functionally, behaviorally, and clinically important population of neocortical neurons. Little is known about their development. “Intratelencephalic” CStrPN (CStrPN_i), projecting to the contralateral striatum, with their axons fully within the telencephalon (intratelencephalic), are a major population of CStrPN. CStrPN_i are of particular interest developmentally because they share hodological and axon guidance characteristics of both callosal projection neurons (CPN) and corticofugal projection neurons (CFuPN); CStrPN_i send axons contralaterally before descending into the contralateral striatum. The relationship of CStrPN_i development to that of broader CPN and CFuPN populations remains unclear; evidence suggests that CStrPN_i might be evolutionary “hybrids” between CFuPN and deep layer CPN—in a sense “chimeric” with both callosal and corticofugal features. Here, we investigated the development of CStrPN_i in mice—their birth, maturation, projections, and expression of molecular developmental controls over projection neuron subtype identity.