Epstein Barr virus (EBV) is a gamma herpesvirus that is associated with several specific lymphoid malignancies, some of which occur more frequently in immunocompromised individuals. EBV infection is almost ubiquitous in healthy adults, so establishing a causal role in lymphomagenesis has been difficult. Support for EBV being an oncogenic virus is derived from its ability to infect and transform normal human B-cells in vitro, resulting in their "immortalization" and leading to continuously growing lymphoblastoid cell lines (LCLs). In addition, viral proteins required for this EBV-mediated transformation have been identified. The presence of EBV in the neoplastic cells of specific lymphoid malignancies is quite consistent, further indicating an etiopathogenic role in their development. Nevertheless, it is clear that while important in the process of lymphomagenesis, infection by EBV is not sufficient. Important co-factors exist for the development of EBV-associated lymphomas, one of which is the lack of normal immune surveillance.