T lymphocytes (T cells) play critical roles in the regulation of immune responses, and are responsible for mediating many of the effector mechanisms of the immune system. For this reason, there has always been a need for assays to measure accurately the activity of populations of T cells, both in model (animal) systems and in humans. The expansion of the biotechnology industry has led to a dramatic increase in the number of novel immunotherapeutics that are being developed for the treatment of cancer, autoimmune disorders and infectious diseases. This increase in activity in the field of immunotherapy, coupled with the expense of clinical trials, has led to renewed interest in methods that accurately assess T-cell function, as researchers seek to maximise the amount of information that can be obtained from each clinical study. Assessing the quantitative and qualitative nature of a T-cell response, for example following vaccination or immunosuppressive therapy, can provide valuable information about the efficacy of a treatment, in place of a clinical endpoint. This article reviews some of the established methods that are used to monitor human T-cell activity, and describes some new approaches that are in development to increase the speed, sensitivity and relevance of such methods.