CD1 is a family of cell surface glycoproteins that are related in structure and evolutionary origin to the major histocompatibility complex (MHC)-encoded antigen-presenting molecules. In contrast to MHC-encoded antigen-presenting molecules, CD1 binds and presents lipid and glycolipid antigens for specific recognition by T cell antigen receptors. Recent work shows that several CD1 family members colocalize with MHC class II proteins within the endocytic system of antigen-presenting cells. Detailed studies of the intracellular trafficking of CD1 proteins reveal new mechanisms controlling delivery of antigen-presenting molecules to particular compartments within cells. The combination of overlapping yet distinct trafficking routes for the various CD1 family members, combined with emerging information on the heterogeneity of CD1-presented lipid antigens, suggest a model whereby different members of the CD1 family could present antigens that occur in various cellular compartments. Furthermore, the CD1 family as a group may present antigens from pathogens that are not normally accessible to or efficiently surveyed by the MHC Class I or II systems. The discovery of this third pathway for antigen presentation, together with the appreciation of a previously unrecognized universe of nonpeptide lipid antigens for T cell responses, are likely to have broad implications for our understanding of the cell-mediated immune response and its role in health and disease.