Some of the key features of aging effects on T lymphocytes have been illuminated in the last few years from new angles. Experimental evidence indicates a profound increase in the proportion of memory versus naive types of T cells, a decline in the response to activation and in the capacity to enter the cycle, and decline in levels of IL-2, yet an increase in various other cytokines. At least part of these changes are based on altered patterns of T lymphocyte development. Long-term cultures of T cells from young donors show that certain manifestations of aging are acquired under such in vitro conditions. The T lymphocyte compartment thus offers a unique experimental model to investigate different checkpoints in development and function of the cells, as based on vulnerability to the effects of aging.