Induction of appropriate types of innate and adaptive immune responses depending on the nature of antigens is crucial to cope with a variety of pathogens and concurrently to avoid pathological reaction to self antigens. Recent studies have been elucidating that the diversity of immune responses is critically controlled by dendritic cells (DCs). Two DC subsets have been identified in humans: myeloid DCs (mDCs) and plasmacytoid DCs (pDCs). The DC subsets recognize different microbial pathogens by expressing distinct repertoires of Toll-like receptors, and induce different types of innate and adaptive immune responses depending on environmental factors. Particularly, pDC precursors produce vast amounts of type I interferons in response to viruses and thus play an important role in anti-viral immunity. In addition, type I interferons derived from pDCs are instrumental in activating mDCs. Elucidating cellular and molecular mechanisms by which functions of the two DC subsets are modulated will lead to understanding the pathogenesis of various immune-related diseases and to developing novel immunological therapies.