When considering preclinical studies to evaluate the safety and immunogenicity of putative vaccine candidates, such as nucleic acid vaccines, species most closely related to humans should be considered. Phylogenetically, the great apes (chimpanzees, orangutans, gorillas, and gibbons) are most closely related to humans. However, the great apes, which diverged from humans over 5 million years ago, represent endangered or threatened species that limits their utility in preclinical studies. In addition, cost considerations for using great apes in biomedical studies represents another serious limitation. The Old World monkeys, (macaques, baboons, mandrills, and mangabeys), diverged from humans over 15 million years ago. A number of the Old World monkey species including rhesus, cynomolgus, and African green monkeys, have also been employed in biomedical research to evaluate vaccine safety and immunogenicity. New World monkeys (aotus, owl, cebus monkeys, and marmosets) are the most phylogenetically divergent from humans, yet they have also been utilized to develop nonhuman primate models for a number of human infectious diseases and tumors. The advantages and disadvantages in selecting a particular nonhuman primate species for studies to evaluate DNA vaccine safety and immunogenicity are briefly discussed. Comparative immunology, reproductive physiology, endogenous infectious agents, and cost considerations are briefly described.