Despite extensively growing knowledge on the molecular biology of ependymoma, effective treatments are still lacking for about half patients with high-risk tumors. By applying single cell RNA-sequencing, we comprehensively identify a cellular hierarchy within ependymoma spanning undifferentiated and differentiated tumor cell states across all major molecular groups. This not only refines the concept of these established molecular groups but moreover provides a biological context for the well-known capacity of aggressive ependymomas for late recurrence and treatment resistance.