The DNA vaccines are simple rings of DNA containing a gene encoding an antigen, and a promoter/terminator to make the gene express in mammalian cells. They are a promising new approach for generating all types of desired immunity: cytolytic T lymphocytes (CTL), T helper cells and antibodies, whilst being a technology that has the potential for global usage in terms of manufacturing ease, broad population administration and safety. This review gives an overview of the mechanisms, preclinical and clinical efficacy of DNA vaccines, and point out the limitations of the first generation of such vaccines, and some of the promising second-generation developments. This technology is also being utilized in the field of proteomics as a tool to elucidate the function of genes. The breadth of applications for DNA vaccines thus ranges from prophylactic vaccines to immunotherapy for infectious diseases, cancer, and autoimmune and allergic diseases.