Cyclic GMP–AMP synthase (cGAS) and stimulator of IFN genes (STING) are cytosolic nucleic acid sensors that detect abnormal presence of cytosolic nucleic acids and hence elicit type I interferon (IFN) responses. The loss of immune homeostasis due to abnormal cGAS–STING signaling pathway is involved in cancer, infectious diseases, autoimmune disorders, as well as cell senescence. Upon cGAS–STING activation, the resulting antitumor and antiviral type I IFN responses have been leveraged for antitumor as well as antiviral therapy. In this chapter, we review the basic biology, current attempts of STING-targeted cancer drug development, as well as drug delivery systems for STING agonists in cancer immunotherapy.