Dendritic cells are potent stimulators of immune responses against foreign antigens. Recent advances in this area include the delineation of distinct developmental pathways for different dendritic cell subsets; the emerging concept that one dendritic cell subset has regulatory functions that may contribute to induction of tolerance to self antigens; increased understanding of the interaction of dendritic cells with microbial products and viruses, such as HIV; and the application of dendritic cells for immunotherapy in certain cancers and possibly for the induction of transplantation tolerance.