Avian tumor viruses are important, economically significant pathogens in the poultry industry, and can provide scientific insight in topics that cannot be experimented in humans. The present review will present viral-induced tumors in poultry and will emphasize the specific findings that can be taken from their study, regarding (a) multiple virus infections in vivo, (b) molecular interactions in vitro and in vivo between avian oncogenic viruses, (c) creation of new viable viruses with altered biological properties by the integration of two retroviruses in vivo, (d) the diversity of clinical signs that appear in viral-infected poultry prior to tumor formation. The inter-viral molecular interaction were studied in vitro and in vivo, in natural infections and in experimentally-infected birds. About 25% of flocks were double-virus-infected, and molecular integrations occurred spontaneously in about 5% of the samples, as by the chimeric molecules detection. In experimentally-infected birds the chimeric molecules rate was higher, while in herpes-and retroviruses co-infected tissue cultures, recombinant virus creation was efficient, leading to a new virus with altered biology. Spontaneous inter-viral recombination occurred between retroviruses, lead to the emergence of a delirious new virus, avian leukosis virus, subgroup J, which actually caused great economic losses to the poultry industry.