Retroviruses1 have been extensively studied during the past 10–20 years. These viruses are of particular interest to molecular biologists for a variety of reasons. Their unique mode of replication (RNA → DNA → RNA) provided the first exception to the central dogma that transfer of genetic information is unidirectional, from DNA to RNA. Retroviruses provide interesting model systems for studying virus-host interactions. They generally do not kill the host cell, and are maintained essentially as cellular genetic elements, integrated in the host chromosome. They contain a small number of genes that are expressed using mainly cellular synthetic mechanisms. Since these genes are expressed at high levels in infected cells, their products can be conveniently assayed and synthetic and processing pathways monitored. Perhaps most intriguing is the ability of many of the retroviruses to transform cells in tissue culture and cause neoplastic disease in animals. Studies of these viruses have led to the identification of more than ten distinct transforming genes and proteins, and have provided important insights into possible mechanisms of oncogenesis.