T-cell exhaustion is a specialized state of T-cell differentiation that arises during persistent antigen exposure and chronic inflammation and represents a major barrier to effective antitumor immunity. Rather than being a homogeneous population, exhausted CD8+ T cells comprise distinct functional and transcriptional states with different capacities for self-renewal, proliferation, cytotoxicity, and response to immunotherapy. Current models describe a developmental continuum from progenitor exhausted T cells (Tpex), through intermediate exhausted states (Texint), to terminally exhausted T cells (Texterm). Tpex cells, typically characterized by TCF1 expression and a stem-like phenotype, retain proliferative capacity and serve as a renewable source of differentiated exhausted cells. In contrast, Texterm cells display high levels of inhibitory receptors, severe functional impairment, and limited proliferative potential. The transcription factors TCF1 and TOX are key regulators of this trajectory. TCF1 promotes the maintenance and expansion of stem-like Tpex cells and is strongly associated with responses to checkpoint blockade, whereas TOX contributes to the establishment and stabilization of exhaustion-associated transcriptional and epigenetic programs. Importantly, TOX and TCF1 function within a complex regulatory network rather than as simple antagonists. Therapeutically, PD-1 blockade primarily expands Tpex populations, which subsequently generate differentiated progeny capable of mediating tumor control. Similarly, CAR-T efficacy is influenced by the differentiation state and exhaustion susceptibility of infused cells. Targeting Tpex preservation, epigenetic regulation, CAR signaling, and the tumor microenvironment may therefore improve the durability and efficacy of cancer immunotherapy.