Elucidation of the organization and operation principles of neural circuits is the key to unlock the mystery of the brain. In the coming decade, single-cell transcriptome analysis will help to identify all cell types and their distribution in rodent, non-human primate (NHP) and human brains. Aided by the transcriptomic information, high-throughput single neuron tracing will yield cell type-specific connectomes of rodent and NHP brains. Circuit-specific neuromodulation will help to elucidate the circuit basis of various brain functions and dysfunctions in NHPs, facilitating the effective use of neuromodulation in treating brain disorders. Future progress could be accelerated by having a consensual definition of neuronal types and subtypes, developing NHP models of brain disorders, and clarifying the meaning of cause and effect within complex interconnected neural networks.