The chapter focuses on simian herpesviruses and the role these viruses play in neoplasia. It describes several simian herpesviruses, along with summarizing data related to this. It compares the oncogenic simian herpesviruses with Epstein-Barr virus (EBV), along with providing additional, indirect evidence for a causal relationship between EBV and some lymphomas in man. The studies of simian oncogenic herpesviruses have led to renewed efforts to transform lymphocytes from animals of the species susceptible to tumor induction by Herpesvirus saimiri (HVS) or Herpesvirus ateles (HVA) with EBV, and to induce lymphomas in some of the same animals. The great importance of nonhuman primate model systems is stated in the chapter for the opportunities they offer in studying the pathogenetic mechanisms of lymphoproliferative diseases induced by herpesviruses. Further use of these models for vaccination and therapeutic studies may lead to the control of these diseases in man.