The dendritic cell system contains conventional dendritic cells(DCs)and plasmacytoid pre-dendritic cells(pDCs).Both DCs and pDCs are bone marrow derived cells.Although the common functions of DCs areantigen-processing and T-lymphocyte activation,they differ in surface markers,migratory patterns,andcytokine output.These differences can determine the fate of the T cells they activate.Several subsets of matureDCs have been described in both mouse and human and the developmental processes of these specialized DCsubsets have been studied extensively.The original concept that all DCs were of myeloid origin was questionedby several recent studies,which demonstrated that in addition to the DCs derived from myeloid precursors,some DCs could also be efficiently generated from lymphoid-restricted precursors.Moreover,it has been shownrecently that both conventional DCs and pDCs can be generated by the Flt3 expressing hemopoietic pro-genitors regardless of their myeloid-or lymphoid-origin.These findings suggest an early developmentalflexibility of precursors for DCs and pDCs.This review summarizes some recent observations on the deve-lopment of DC system in both human and mouse.Cellular & Molecular Immunology.2004;1(2):112-118. The dendritic cell system contains conventional dendritic cells (DCs) and plasmacytoid pre-dendritic cells (pDCs). Although the common functions of DCs are antigen-processing and T-lymphocyte activation, they differ in surface markers, migratory patterns, andcytokine output. these differences can determine the fate of the T cells they activate. Several subsets of matureDCs have been described in both mouse and human and the developmental processes of these specialized DCsubsets have been studied extensively.The original concept that all DCs were of previously mytioned origin was questioned by several recent studies, which form that in addition to the DCs derived from myeloid precursors, some DCs could also be efficiently generated from lymphoid-restricted precursors. Moreover, it has been shown monstrously that both conventional DCs and pDCs can be generated by the Flt3 expressing hemopoietic pro-genitors regardless of their myeloid-or lymphoid-origin.These fin dings suggest an early developmental flexibility of precursors for DCs and pDCs. This review summarizes some recent observations on the deve lopment of DC system in both human and mouse. Cellular & Molecular Immunology. 2004; 1 (2): 112-118.