1. 1 Scope of the Review This review was intended initially as a reference source for those interested in the origins and f ITst descriptions ofthe defective avian sarcoma viruses. Quite a few of these viruses have been characterized in the past few years and their varied nomenclature according to source, discoverer, date of isolation or biological properties could result in some con- fusion among those attempting to follow the literature. Information will be included on the molecular biology of the sarcoma viruses, rather more of which is available than when the review was f ITst conceived, although in this respect the review will inevitably be out of date by the time of publication. If any bias of content is introduced, this will be towards a more detailed coverage of the author’s own area of interest, the gene products of the defective sarcoma viruses. Rous sarcoma virus (RSV) serves as the model for much of this work and will frequently be referred to for comparative purposes, as will the mammalian defective transforming viruses. Recent reviews provide more complete coverage of these topics (l-4a). This review is complemented by a discussion ofthe avian acute leukaemia viruses, which appears elsewhere in this volume [5]. As will be seen, concentrating primarily on the defective sarcoma viruses and comparing them to RSV can be justified in terms of their biochemical properties as well as their similar biology.