Dendritic cells (DCs) control the initiation and differentiation of T cells. In the steady state, DCs mediate tolerance. To achieve immunization, the tolerogenic function of DCs must be switched off by inducing their maturation with appropriate "adjuvants." Dendritic cells form a system composed of distinct subsets that differ in their expression of endocytic and signaling receptors. These subsets have different capacities to differentiate and polarize T cells and to cross-present antigen to expand CD8+ T cells. Optimization of vaccines is possible by exploiting the unique biological properties of DCs.