The innate immune system deploys a variety of sensors to detect signs of infection. Nucleic acids represent a major class of pathogen signatures that can trigger robust immune responses. The presence of DNA in the cytoplasm of mammalian cells is a danger signal that activates innate immune responses; however, how cytosolic DNA triggers these responses remained unclear until recently. In this review, we focus on the mechanism of DNA sensing by the newly discovered cGAS-cGAMP-STING pathway and highlight recent progress in dissecting the in vivo functions of this pathway in immune defense as well as autoimmunity.