Mammalian innate immunity possesses a distinct system to recognize aberrant DNA inside the cell. One class of DNA sensors is the Toll-like receptor 9, which is expressed in the specialized immune cells, binds to single-stranded DNA in the endosome to transmit cellular signaling through myeloid differentiation primary response protein 88 (MyD88). Another class of DNA sensors exists in the cytoplasm of most type of cells in the tissue, detecting double-stranded DNA to signal through TANK-binding kinase-1 (TBK1)-mediated type-I interferon production and apoptosis-associated speck-like protein containing a CARD (ASC)-mediated IL-1β secretion. Since DNA sensors have potential to recognize aberrant DNA of both self and nonself origin, their physiological roles in microbial infection, tissue damage, autoimmune diseases, and DNA-based therapeutic applications are being intensively investigated.