The phylogenic analysis of the nucleotide sequences of the env gene has enabled classification of HIV-1 into three groups. The group M of HIV-1 infection has been classified into 9 different genetic subtypes A-K, with E and I being classified as circulating recombinants forms (CRFs). The groups O and N are less frequently encountered in human infections. Presently group M of HIV-1 globally causes 99.6 per cent of all human infections. The epidemiological trends suggest that subtype C strains would dominate the HIV pandemic in the coming years. The geographic spread of subtype C strains is also very diverse with prevalence in Africa, Latin America and Asia. Data from India show a high prevalence of subtype C. In north and western India, 78.4 and 96 per cent of HIV-1 strains respectively were shown to be subtype C. Among female sex workers in Kolkata 95 per cent of the HIV-1 strains were subtype C. The south Indian subtype data are very similar to the data from the rest of India. The HIV-2 groups (subtypes) recognized are A-H. Unlike HIV-1, HIV-2 strains are predominantly found in Africa. The Indian HIV-2 strains identified till date are subtype A. This is also the predominant strain circulating in western African countries. This group (subtype) is estimated to cause 0.11 per cent of all HIV infections in humans.