Three species of avian retrovirus cause disease in poultry: the avian leukosis/sarcoma virus (ALSV), reticuloendotheliosis virus (REV), and lymphoproliferative disease virus (LPDV) of turkeys. The ALSV can be classified as slowly transforming viruses, which lack a viral oncogene, and acutely transforming viruses, which possess a viral oncogene. Slowly transforming viruses induce late onset leukoses of the B cell lymphoid, erythroid, and myeloid cell lineages, and other tumors, by viral promoter insertion into the genome of a host cell and activation of a cellular protooncogene. The various acutely transforming leukemia and sarcoma viruses induce leukotic or other tumors rapidly and carry one or anther (sometimes two) viral oncogenes, of which some 15 have been identified. The ALSV fall into six envelope subgroups, A through E, and the recently recognized J subgroup, which induces myeloid leukosis. With the exception of Subgroup E viruses, these viruses spread vertically and horizontally as infectious virions, and are termed exogenous viruses. Subgroup E viruses are usually spread genetically as DNA proviruses (often defective) in host germ cell genome, and are termed endogenous viruses. Several other families of endogenous viruses also exist, one of which, endogenous avian retrovirus (EAV), is related to Subgroup J ALV. Exogenous viruses, and sometimes endogenous viruses, can have detrimental effects on commercially important production traits. Exogenous viruses are currently controlled by virus eradication schemes. Reticuloendotheliosis virus, which lacks a viral oncogene, causes chronic B cell and T-cell lymphomas in chickens, and also chronic lymphomas in turkeys and other species of birds. An acutely transforming variant of REV, Strain T, carries a viral oncogene, and induces reticuloendotheliosis within a few days. In chickens and turkeys, REV spreads vertically and horizontally. No commercial control schemes are operated. In turkeys, LPDV infection has occurred in several countries, where it caused a lymphoproliferative disease of uncertain nature.