An extraordinary variety of neuronal subtypes, each with distinct morphologies, patterns of connectivity, and electrophysiological properties, is generated during neocortical development. Elucidating programs of molecular controls that govern progressive specification of neuronal subtype identity in the cerebral cortex contributes toward our understanding its development, organization, evolution, and function. Establishing a basic framework for how and when cell fate specification decisions are made will enable more efficient manipulation of these transitions in vitro, and therefore has important practical implications for biomedical research, as it will help improve protocols for generation of specific cortical neuron subtypes for use in disease modeling, drug screening, and therapeutic transplantation.