HIV is the member of lentivirus, subfamily of retroviruses which affects humans. The most common cause of AIDS throughout the world is HIV 1 and 2. HIV is a spherical enveloped virus, having nucleocapsid that has ribonucleoproteins. HIV 1 also has electron dense core that contain several proteins, 2 strands of RNA and the enzyme reverse transcriptase. The viron also contains GAG, POL and ENV genes that codes for core protein, reverse transcriptase and envelope proteins respectively. Both HIV 1 and 2 have 5 other non structural genes (except 3 structural genes) – TAT, REV, NEF, VIF and VPR. Other then this HIV 1 contains VPU and HIV 2 has VPX. Product of these genes acts as antigens and infected person’s serum contain antibodies to these antigens. Detection of them is of great importance in the diagnosis and prognosis of HIV infection. HIV is a highly mutable virus. Not only there are differences between isolates of HIV from different places or persons but also between sequential isolates from the same person and even between those obtained from different sites of the same person at the same time. HIV classified into 2 major groups “M” and “O”. “M” contains ten genetically distinct subtypes (A, B, C, D, E, F, G, H, I, J) and “O” contains several heterogeneous viruses. The dominant subtype in western India is C while other subtypes are A, B, F. Subtypes seem to vary in the case of transmissibility by different routes.