Genes can be delivered into cells in vivo primarily by two methods: as a part of the viral (or retroviral) genome or as a bacterial plasmid. Viral vectors with reconstituted gene and depleted viral proteins use viral transfection pathways, while bacterial plasmids are not infectious per se and must be inserted into cells by appropriate gene delivery systems. We shall briefly discuss viral vectors in the next chapter and will mostly concentrate on the design of plasmids.