T memory stem (TSCM) cells are a rare subset of memory lymphocytes endowed with the stem cell-like ability to self-renew and the multipotent capacity to reconstitute the entire spectrum of memory and effector T cell subsets. Cumulative evidence in mice, nonhuman primates and humans indicates that TSCM cells are minimally differentiated cells at the apex of the hierarchical system of memory T lymphocytes. Here we describe emerging findings demonstrating that TSCM cells, owing to their extreme longevity and robust potential for immune reconstitution, are central players in many physiological and pathological human processes. We also discuss how TSCM cell stemness could be leveraged therapeutically to enhance the efficacy of vaccines and adoptive T cell therapies for cancer and infectious diseases or, conversely, how it could be disrupted to treat TSCM cell driven and sustained diseases, such as autoimmunity, adult T cell leukemia and HIV-1.