Recent advances in high-throughput single-cell transcriptomics and epigenomics methods allow to measure the expression of thousands of genes and identify the gene transcriptional regulation sites in large number of single cells. The scale and high information content of the advanced single-cell omics methods now, for the first time, promise to define all or most cell types and states at molecular level in human and other mammalian brains. The molecular taxonomies are the first step of the BICCN toward establishing comprehensive brain cell reference atlases, to facilitate developing cell-specific targeting tools to modulate brain circuits in controlled manner and understanding molecular and cellular mechanisms of brain disorders. Seven BICCN investigators will present comprehensive cell census results obtained from mouse, human, and monkey brains in complementary manner.