Epstein-Barr virus (EBV) is a gammaherpesvirus and is widespread in all areas of the world, infecting over 95% of the adult population. EBV primarily infects and replicates in the stratified squamous epithelium of the oropharynx during acute infection. Besides, its well-known tropism for B cells, the targets of EBV infection may also include epithelial cells, T cells, and cells of the macrocytic, granulocytic, and natural killer lineages. Although most humans coexist with the virus without serious sequelae, a small proportion will develop tumors. Almost every undifferentiated nasopharyngeal carcinoma (NPC) is EBV positive, despite geographical origin. EBV-derived IL-10 which is considered to play a role in the establishment of latent infection by suppression of the host immune system, may contribute to the growth of the tumor and to immune evasion. Latent membrane protein-1 (LMP-1) has transforming ability and support to the concept that EBV is involved in the pathogenesis of NPC. The association of NPC with EBV has been firmly established however the evidence indicating a role for the virus in the pathogenesis is still unknown and controversial. The main question is how the EBV-infected cells can escape from the immune response. [Turk J Cancer 2006;36(3):97-107].