Dendritic cells (DCs) are professional antigen presenting cells, which display an extraordinary capacity to stimulate naı̈ve T cells and initiate primary immune responses. This established function of DCs has now offered the hope to apply DC-based immunotherapy for cancers. Recent studies suggest that DCs also play critical roles in the induction of peripheral immunological tolerance, regulate the types of T cell immune responses, and function as effector cells in innate immunity against microbes. The diverse functions of DCs in immune regulation depend on the diversity of DC subsets and lineages, and on the functional plasticity of DCs at the immature stage.