NKT cells are a population of T cells that have a broad range of regulatory effects on the immune system. Somewhat paradoxically, they can both suppress and potentiate cell-mediated immune responses; for example, while they can suppress some autoimmune diseases, they can also promote potent tumour rejection. There is accumulating evidence to suggest that this functional dichotomy can be explained by the existence of functionally distinct NKT cell subsets, which can differentially regulate the behaviour of other immune cells and drive remarkably different outcomes in disease settings. / Studies in vivo and in vitro have demonstrated remarkable functional diversity between NKT cell subsets of different phenotypes, and deriving from different tissues. This thesis examined functional differences between NKT cell subsets in the context of type 1 diabetes in NOD mice, identified a phenotypically and developmentally distinct IL-17 producing NKT cell subset, and investigated the functional effects of thymic NKT cells on the development and maintenance of conventional T cells and thymic stromal cells. The data presented in this thesis adds to the accumulating evidence that NKT cells are a functionally heterogeneous population, and reiterates the important point that subsets of NKT cells should be studied separately in order to properly understand the biological function of this important regulatory T cell population, and to maximise their clinical potential.