Two genetically different but related forms of HIV, called HIV-1 and HIV-2, have been isolated from patient with AIDS and also closely related viruses are found in many species of non-human primates. Significant progress has been made in preclinical and clinical research in the worldwide efforts to develop an effective HIV vaccine. Many approaches like attenuated and whole inactivated vaccine products, along with recombinant DNA, protein and peptide approaches and some novel construct including other lentivirus have been studied in various animal models of HIV infection. Currently, 2 gp160 vaccines, 2 gp120 products, 1 envelope yeast-derived protein and 1 vaccinia gp160 recombinant vaccine has entered the clinical trials in United States and gag particle product has been studied in the United Kingdom. No immunization strategy has been demonstrated to be completely effective in preventing HIV infection in vivo. Major scientific obstacles blocking the development of a successful preventive vaccine are the extraordinary variability of HIV, the lack of exact animal model of HIV-induced AIDS and understanding of the correlates of positive immunity to HIV.