During infection, naive CD8(+) T cells differentiate into effector cells, which are armed to eliminate pathogens, and memory cells, which are poised to protect against reinfection. The transcriptional program that regulates terminal differentiation into short-lived effector-memory versus long-lived memory cells is not clearly defined. Through the use of mice expressing reporters for the DNA-binding inhibitors Id2 and Id3, we identified Id3(hi) precursors of long-lived memory cells before the peak of T cell population expansion or upregulation of cell-surface receptors that indicate memory potential. Deficiency in Id2 or Id3 resulted in loss of distinct CD8(+) effector and memory populations, which demonstrated unique roles for these inhibitors of E-protein transcription factors. Furthermore, cytokines altered the expression of Id2 and Id3 differently, which provides insight into how external cues influence gene expression.