The review discusses the differentiation of memory CD8 T cells during viral infections, focusing on the three characteristic phases: initial activation and expansion, the death phase, and the formation of memory T cells. It highlights the dramatic cellular and transcriptional changes that occur during each phase, leading to the acquisition of effector functions and the survival of a small fraction of effectors. The review also explores the mechanisms underlying the transition from effector to memory T cells, including the role of homeostatic signals such as IL-7 and IL-15. Additionally, it examines the functional changes in memory CD8 T cells, particularly the hierarchical loss of effector functions during chronic infections, and the impact of viral load and CD4 help on T-cell responses. The implications of these findings for vaccine design are discussed, emphasizing the importance of inducing memory T cells with high proliferative potential and understanding the nature of defects in exhausted T cells for effective therapeutic interventions.The review discusses the differentiation of memory CD8 T cells during viral infections, focusing on the three characteristic phases: initial activation and expansion, the death phase, and the formation of memory T cells. It highlights the dramatic cellular and transcriptional changes that occur during each phase, leading to the acquisition of effector functions and the survival of a small fraction of effectors. The review also explores the mechanisms underlying the transition from effector to memory T cells, including the role of homeostatic signals such as IL-7 and IL-15. Additionally, it examines the functional changes in memory CD8 T cells, particularly the hierarchical loss of effector functions during chronic infections, and the impact of viral load and CD4 help on T-cell responses. The implications of these findings for vaccine design are discussed, emphasizing the importance of inducing memory T cells with high proliferative potential and understanding the nature of defects in exhausted T cells for effective therapeutic interventions.