The expansion of the neocortex, a hallmark of mammalian evolution^(1,2), was accompanied by an increase in cerebellar neuron numbers³. However, little is known about the evolution of the cellular programs underlying cerebellum development in mammals. In this study, we generated single-nucleus RNA-sequencing data for ~400,000 cells to trace cerebellum development from early neurogenesis to adulthood in human, mouse, and the marsupial opossum. We established a consensus classification of …