Dendritic cells form a system of leukocytes specialized to stimulate resting T cells in vivo. Dendritic cells are crucial for the initiation of primary immune responses of both helper and cytotoxic T lymphocytes, and thus act as 'nature's adjuvant'. The manifold specializations underlying this in vivo immunostimulatory function are becoming increasingly clear. Methods have been developed to generate large numbers of dendritic cells from hematopoietic precursors in vitro. These techniques now allow molecular studies as well as the use of antigen-charged dendritic cells to vaccinate patients against tumors (e.g. B-cell lymphoma or melanoma) and infection (e.g. HIV-1). Recent data suggest that besides the classical immunostimulatory dendritic cells which belong to the myeloid lineage, there exist regulatory dendritic cells related to the lymphoid lineage. These lymphoid-derived dendritic cells which at least in part express Fas-ligand appear to be involved in the induction of central as well as peripheral tolerance, and in the future might allow a novel approach to induce tolerance in transplantation, autoimmunity, and allergy.