Evidence including genome-wide analyses have uncovered that epigenetic mechanisms regulate differentiation and effector functions in CD8 T cells. Gene expression profiles change when CD8 T cells differentiate from naïve T cells to memory T cells. It has been shown that this programmed differentiation is regulated by epigenetic mechanisms. Upon antigen stimulation, CD8 T cells activate and acquire effector functions to target cells. Effector molecule gene expressions are upregulated by epigenetic mechanisms in CD8 T cells. It is suggested that memory T cells respond more rapidly to antigens because chromatin structures of effector molecule genes are open and their gene transcriptions are poised for activation.