Over the past 10 years a new population of cells has been the focus of much attention. The functions of these unique lymphocytes, characterized by the concomitant expression of T- and NK-cell markers and thus termed NKT cells, have been implicated in many diverse aspects of immunity, including regulation of autoimmune disorders, control of tumour growth and spread, and defence against a number of pathogens. Although much debate still remains as to the natural role of NKT cells, it is clear that these cells have the capacity, either constitutively or postactivation, to promote an amazing array of immunoregulatory responses. The involvement of NKT cells in viral immune-surveillance and their ability to induce protection against pathogens once activated make them an attractive clinical target.