In this week’s issue, researchers working as part of the BRAIN Initiative Cell Census Network (BICCN) present nine papers that map the entire mouse brain in unprecedented detail. Three papers by Hongkui Zeng, Xiaowei Zhuang, Evan Macosko and colleagues describe the transcriptomic and spatial cell atlases of the whole mouse brain, depicting the molecular signatures, spatial organization, and cell–cell interactions of thousands of cell types. (A related paper published earlier by Xiao Wang and co-workers also probes the cell type architecture in the brain.) A pair of papers by Edward Callaway, Joseph Ecker, Zhigang He and colleagues examine the links between the identity of neurons and where they extend their axons. And across three papers, Bing Ren, Joseph Ecker and co-workers examine the epigenetic landscape of the mouse brain, looking at regulatory elements in various cell types. Finally, Karthik Shekhar, Joshua Sanes and colleagues use single-cell RNA sequencing data from 17 vertebrate species to probe the evolution of cell types in the retina.