NKT cells are a heterogeneous subset of T lymphocytes that share surface markers and functional characteristics with both conventional T lymphocytes and natural killer cells. Most NKT cells express a semi-invariant T cell receptor that reacts with glycolipid antigens presented by the CD1d molecule on the surface of antigen-presenting cells. NKT cells can modulate the immune response against infectious agents, autoantigens, tumors, tissue grafts and allergens. NKT cells mediate the activities through their ability to express pro- and anti-inflammatory cytokines that influence the type and magnitude of the immune response. The manuscript summarizes current views on development of NKT cells as well as mechanisms and effects of their action.