Reticuloendotheliosis is a neoplastic disease of poultry caused by a group of reticuloendotheliosis viruses (REVs) belonging to the genus Gammaretrovirus, family Retroviridae. Affecting a variety of avian species such as chicken, turkey, duck, goose, pheasant, and peafowl, reticuloendotheliosis is characterized by immunosuppression, runting-stunting syndrome, chronic lymphomas, and chronic neoplasm, which resemble both Marek’s disease and avian leukosis. Interestingly, the etiologic agent for Marek’s disease is Marek’s disease virus (MDV) of the genus Alphaherpesvirus, family Herpesviridae, and that for avian leukosis is avian leukosis virus (ALV) of the genus Alpharetrovirus, family Retroviridae. These avian oncogenic viruses have a huge economic impact on poultry industry worldwide, and their overlapping disease proles make differential diagnosis on clinical ground difcult, if not impossible. As the topics on MDV and ALV are covered in Chapters 17 and 87, the main focus here is REV.