As T lymphocytes proliferate and differentiate in vivo or in vitro, their functional capacity can change dramatically. In particular, extensive cell division is often associated with telomere shortening and the onset of cellular senescence, thus impacting the proliferative potential of the cells. Telomere length and integrity represent therefore key molecular markers of the status and aging of the cells. To assess these markers, we established qPCR-based methods to measure telomere length as well as telomerase activity, applied to low cell numbers, which is necessary when working with rare or small subsets of T lymphocytes.