Infection by exogenous avian leukosis viruses (ALVs) causes economic loss from neoplastic mortality and from impaired performance of subclinically infected chickens. This paper reviews progress in research related to natural infection and its control. Subgroup A ALVs causing lymphoid leukosis are the most common viruses in the field, but variant viruses can arise and cause losses. In Israel in recent years, epidemic outbreaks of haemangiosarcomas caused by a virus of unusual cytopathogenicity have occurred. In the UK, an ALV belonging to a new subgroup for chickens has been recently isolated; this virus is able to cause myeloid leukosis and nephromas. ALV infection of commercial stock is controlled by virus eradication schemes that prevent vertical transmission of ALV from one generation to the next. In this regard, endogenous leukosis viruses and ev loci, notably ev21 linked to the K slow-feathering gene, have been shown to have a detrimental influence on responses to infection by exogenous ALVs, and on the success of eradication schemes. Tolerogenic properties of the ev loci are involved. Attention has also been directed to whether ev loci have any direct influence on performance traits. Although eradication provides the main means of controlling ALV, the development of transgenic techniques in chickens has renewed interest in genetic resistance, and some progress has also been made in developing recombinant vaccines.