Phylogenetic study of HIV-1 strains from different geographical locations has revealed the existence of three separate groups that have been named M, N, and O. Most strains involved in the pandemia belong to group M which contains several subtypes. Nearly 20 p. 100 of isolates in group M are recombinant with genomic components from different subtypes. Some mosaic virus are one-of-a-kind or limited to small transmission groups while others are major players in the worldwide AIDS epidemic. They are currently called circulating recombinant forms (CRFs). Since subtypes or CRFs must be similar over the whole genome, only 9 subtypes are possible within group M (A, B, C, D, F, G, H, J and K). Viruses E and I in the envelope are recombinant. Subtyping is a powerful molecular tool for monitoring the evolution of the HIV-1 epidemic. Overall the predominant viral forms in the world are subtypes A and C followed by recombinant CRF01-AE (formerly subtype E) and CRF02-AG (identical to the IBNG prototype strain in Nigeria) and subtype B. The highest degree of genetic diversity in HIV-1 is observed in Africa where all subtypes and groups can be observed. The geographic distribution of subtypes is subject to constant change. Recombinant forms of the virus will continue to appear as long as the different subtypes of HIV-1 continue to circulate between continents and recombination continues to occur.