Abstract: The septum is ventral forebrain structure responsible for the regulation of emotional states including anxiety, fear and depression. The septum contains an extremely diverse array of short- and long-range projecting GABAergic neurons distributed across its two histological subdivisions, the medial and lateral septal nuclei. Both of these regions are heavily interconnected with other brain areas through efferent and afferent projections. It is currently unclear how septal neuronal diversity and the wiring of its circuits are specified during development. Using molecular genetics and single-cell/nuclei RNA-seq we have begun to unravel the developmental logic for producing diverse neural cell types in the septum. Our future work is focused on understanding the specific contribution of developmentally specified neural cell types to the regulation of internal states carried out by the septum.