The commercially important neoplastic diseases of poultry are Marek's disease, which is caused by a herpesvirus, and the avian leukoses and reticuloendotheliosis, which are caused by retroviruses. These diseases are responsible for economic loss due to both mortality and depressed performance. Marek's disease virus (MDV) and avian leukosis viruses (ALVs) are prevalent throughout the world, and new strains which arise in particular locations may spread across borders, thereby undermining national disease control measures. Reticuloendotheliosis virus (REV) is also present in many countries. Marek's disease virus is transmitted horizontally only, and international spread in hatching eggs and day-old chicks can be prevented by appropriate hygiene precautions. Transmission of ALV and REV occurs both horizontally and vertically (through the egg), and measures to prevent international spread are more demanding. Marek's disease is controlled by vaccination, whilst avian leukosis is controlled by virus eradication programmes, mainly at the primary breeding level. Similar virus control measures can be applied for reticuloendotheliosis if necessary. No strong evidence exists to suggest that these avian tumour viruses constitute a danger to public health.