In just a few years, injection of plasmid DNA to elicit immune responses in vivo has developed from an interesting observation to a viable vaccine strategy. DNA vaccines have been shown to elicit both cellular and humoral immune responses and to be effective in a variety of preclinical bacterial, viral, and parasitic animal models. This review will discuss the current knowledge of vector design, methods of plasmid delivery, immune responses elicited by various DNA vaccines, safety issues, and production and release of plasmid as a vaccine product. The potential of this new vaccine strategy and its future prospects is summarized.