In this second volume in the series The Retroviridae, the readers are treated to up-to-date reviews on these viruses, which are found in a variety of animal species. The volume begins with important observations on the general fea- tures of retrovirus entry into cells as determined by the viral envelope glyco- proteins and the cell surface receptors (Chapter 1). Aspects of this virus-cell interaction form the basis for the variety of biologic effects associated with this virus family. A timely review of the oncogenic feline viruses is included next (Chapter 2). These viruses, along with the avian and murine retroviruses (Volume 1, Chapters 6 and 7), have provided valuable insight into cancer induction in other animal species, including humans. Two other major groups of retroviruses, the lentiviruses and the spuma- viruses, are introduced in the present volume. Initially these groups, along with oncoviruses, were the three subfamilies in the Retroviridae. This com- plex virus family has now been reclassified into seven separate genera (Vol- ume 1, Chapter 2). Equine infectious anemia virus is the prototype for a lentivirus (Chapter 5). One of the first viruses discovered in nature (1904), it was only relatively recently recognized as a retrovirus. Lentiviruses are asso- ciated primarily with immunologic disorders such as autoimmunity and AIDS (Chapters 3-5), but their association with neurologic disease is also well established (Chapters 4 and 5, and future volumes).