Ideal vaccines are expected to give lifetime protection from infectious diseases, and if possible, from allergic diseases, autoimmune diseases and cancer. DNA vaccination was introduced two decades ago as a simple plasmid inoculation method with a capability of inducing both cellular and humoral immune responses. Recent studies have provided insights into the molecular mechanisms by which the double-stranded structure of DNA vaccine induces the activation of type-I interferon (IFN)-mediated innate immune responses via STING/TBK1 complex, similar to cytosolic double stranded DNA (dsDNA) recognition of immune cells. In this chapter, DNA vaccines and the current knowledge on their mechanism of action will be introduced. The possibilities of using this knowledge for improving immunogenicity of DNA vaccines in humans will then be discussed.