The phenotypic and functional studies carried out in recent years on dendritic cells (DC) purified from spleen and lymph nodes has revealed the existence of heterogeneous populations with distinct life cycles, migratory properties and antigen presenting functions. A major subdivision can be made between "tissue derived" DC that migrate to the lymph nodes from peripheral tissues, both in the steady state and in the course of infections, and "blood-derived" DC, which reside in the spleen and lymph nodes throughout their life cycle. These two groups of DC can be subdivided into smaller subsets. The tissue-derived and the blood-derived DC also show fundamental differences in maturational status and antigen presenting capabilities. In this review, we summarize the roles played by the different DC types in the steady state and during pathogen infections, relating those roles to maintenance of peripheral tolerance and the induction of immunity. We point out the caveats of assuming that the DC that collect antigens are the ones involved in their presentation, emphasizing the phenomenon of antigen transfer as an important component of the immune response.