The locally projecting GABAergic interneurons of the mammalian cerebral cortex are a highly heterogeneous population, whose malfunction or deficit has been implicated in a wide range of neurological disorders. However, the low incidence of the various distinct interneuron populations within the neocortex, combined with the lack of molecular or physiological markers specific to these subtypes, have hampered investigations into their function in the normal and dysfunctional brain. A number of research groups have begun to elucidate the developmental genetic mechanism that underpins this diversity in the mouse neocortex, spurred on by the knowledge that the temporal and spatial origin of an interneuron in the embryonic brain is predictive of its eventual intrinsic properties in the mature cortex. In this review we highlight a number of recent findings that strengthen our understanding of the transcription factor code that is at the heart of generating this diversity. Further understanding of this code will enable selective observation, targeting and manipulation of interneuron subtypes across both in vitro and in vivo systems.