Understanding and manipulating CD8 T cell exhaustion is of fundamental importance for improving T cell-based treatments against chronic infectious diseases and malignant tumours. Within the scope of this lead agency project, jointly funded by DFG and SNF, we have identified a novel master regulator of CD8 T cell exhaustion, the transcription factor TOX. This master regulator is required for the generation and long-term maintenance of exhausted CD8 T cell populations. Deletion of its DNA binding domain leads to a marked decline in the number of exhausted T cells induced by Cl13 infection—most pronounced in the TCF-1+ compartment of Tmex cells. Further on, we have shown that, in contrast to classical memory CD8 T cells, these Tmex cells do not require CD4 T cell help for their maintenance but only for their differentiation into more cytolytically-active effector subsets. Finally, we have used single-cell fate mapping to show that Tmex cells indeed harbour a stem-cell like capacity for self-renewal and multipotency but are imprinted early-on during chronic Cl13 infection with an exhausted differentiation program, which they faithfully maintain and pass on to their descendants even upon reactivation via acutely resolving Arm infection.