Immunosenescence contributes to render older adults more susceptible to pathogens and to develop main age-related diseases. With the ever-increasing rise in the number and proportion of older persons worldwide, one of the challenges of a "long-life society" is to ensure that the years gained are not only healthy and disability-free years but also offering a good quality of life. In this perspective, recent research has concentrated on identifying the factors hindering healthy ageing, and among them, the age-related changes of the immune system have thus been particularly investigated. With respect to its central role in orchestrating the immune response, this chapter describes the main features of T cell-mediated immune senescence and the underlying mechanisms contributing to this state of increased vulnerability. Furthermore, it will explore the means by which T cell functions could be identified by using biomarkers.