Invariant natural killer T (iNKT) cells are a subset of T lymphocytes that react with glycolipid antigens presented by the major histocompatibility complex class I-related glycoprotein CD1d. Although iNKT cells express an antigen-specific receptor of the adaptive immune system, they behave more like cells of the innate immune system. A hallmark of iNKT cells is their capacity to produce copious amounts of immunoregulatory cytokines quickly after activation. The cytokines produced by iNKT cells can influence the level of activation of many cell types of the innate and adaptive immune systems as well as the quality of an adaptive immune response. As such, iNKT cells have emerged as important regulators of immune responses, playing a role in microbial immunity, autoimmunity, tumor immunity, and a variety of inflammatory conditions. Although several endogenous and exogenous glycolipid antigens of iNKT cells have been identified, how these glycolipids orchestrate iNKT-cell functions remains poorly understood. Nevertheless, iNKT cells hold substantial promise as targets for development of vaccine adjuvants and immunotherapies. These properties of iNKT cells have been investigated most extensively in mouse models of human disease using the marine sponge-derived agent alpha-galactosylceramide (alpha-GalCer) and related iNKT-cell antigens. While these preclinical studies have raised enthusiasm for developing iNKT-cell-based immunotherapies, they also showed potential health risks associated with iNKT cell activation. Although alpha-GalCer treatment in humans was shown to be safe in the short term, further studies are needed to develop safe and effective iNKT-cell-based therapies.