Lentiviruses are a family of retroviruses linked by similarities in genetic composition, molecular mechanisms of replication and in biological interactions with their hosts. They are best known as agents of slow disease syndromes that begin insidiously after prolonged periods of subclinical infection and progress slowly leading to the degeneration of multiple organ systems, cachexia and death. The viruses are species-specific in host range and several have been recognized as pathogens of domestic animals, non-human primates and humans. The prototypes of the family are the agents causing maedi-visna in sheep and infectious anaemia in horses. These diseases have been known for several decades and studies on the biology of the viruses have provided a fund of information that predicted most of the properties of their human counterpart which was identified only 6 years ago as the aetiological agent of AIDS. Lentiviruses persist indefinitely in their hosts and replicate continuously at variable rates during the course of the lifelong infection.