The neural assembly underlying the formation of functional networks in the cerebral cortex is conceivably the most complex biological system that exists. Much of this complexity arises during development through the interaction of dozens of different neuronal populations, which belong to two general classes: excitatory glutamatergic pyramidal cells and inhibitory gamma-aminobutyric containing (GABAergic) interneurons. Perhaps the most fascinating aspect of the assembly of cortical circuits is that pyramidal cells and interneurons are generated in distant germinal zones. Pyramidal cells are born locally from progenitors located in the cortical anlage, while interneurons derive from progenitors in the embryonic subpallium. Much progress has been made recently in understanding the molecular mechanisms that regulate the migration of interneurons towards the cortex, but how interneurons find their appropriate partners to build cortical networks with balanced excitation and inhibition remains an enigma.The general goal of this project is to identify the mechanisms controlling the precise allocation of different classes of interneurons into specific layers of the cortex, where they assemble into neural circuits. We also aim to determine how the allocation of interneurons into specific cortical layers influences their function. This project is now possible due to the unique combination of our detailed know-how on the early development of cortical interneurons, including a variety of genetically modified mice available to us, and the application of new technologies to specifically target synchronically generated populations of interneurons. Our multidisciplinary approach, combining mouse genetics, in vivo functional genomics and electrophysiological methodologies represents a technological breakthrough that should accelerate our understanding of the general principles guiding the assembly of neuronal circuits in the cerebral cortex.