Feline immunodeciency virus (FIV) is a retrovirus that is responsible for an immune disorder in cats throughout the world. Clinically, FIV infection is characterized by an acute phase with fever, leucopenia, gingivitis, and generalized lymphadenopathy, followed by a latent phase without obvious symptoms, and a last, symptomatic phase with CD4+ lymphocyte depletion and increased susceptibility to opportunistic infections and neoplasia. Given the occurrence of two other retroviruses in cats, that is, feline leukemia virus (FeLV) and feline foamy virus (FeFV), that induce similar clinical symptoms to FIV, it is important that FIV infection is accurately diagnosed for implementation of appropriate disease management and prevention. The purpose of this chapter is to present a brief overview on FIV relating to its classication, morphology, genome organization, epidemiology, clinical features, pathogenesis, treatment, and prevention, together with a streamline molecular protocol for FIV identication.