The genetic modification of T lymphocytes is an important approach to investigating normal T-cell biology and to increasing antitumour immunity. A number of genetic strategies aim to increase the recognition of tumour antigens, enhance antitumour activities and prevent T-cell malfunction. T cells can also be engineered to increase safety, as well as to express markers that can be tracked by non-invasive imaging technologies. Genetically modified T cells are therefore proving to be of great value for basic immunology and experimental immunotherapy.