A major goal of neuroscience is to understand how circuits of neurons and non-neuronal cells process sensory information, generate movement, and subserve memory, emotion and cognition. Elucidating the properties of neural circuits requires an understanding of the cell types that comprise these circuits and their roles in processing and integrating information. However, since the description of diverse neuronal cell types over a century ago by Ramon y Cajal, we have barely scratched the surface of understanding the diversity of cell types in the brain and how each individual cell type contributes to nervous system function. Historical approaches for classifying neurons rely upon features including the differential expression of small numbers of genes, cell morphology, anatomical location, physiology, and connectivity – important descriptive properties that nonetheless are insufficient to fully describe or predict the vast number of different cell types that comprise the mammalian brain. This NIH-supported BRAIN Initiative project – a collaboration between the Ngai lab and 4 other research groups at UC Berkeley and part of the larger BRAIN Initiative Cell Census Network – is focused on developing single cell RNA sequencing approaches for identifying and classifying the diverse cell types in the mammalian nervous system and CRISPR/Cas9 gene editing techniques for genetically targeting newly discovered cell types.