Feline leukemia virus (FeLV) is a gammaretrovirus that occurs worldwide in domestic cats, as well as small wild cats. It is associated with various serious, and sometimes fatal, diseases including anemia, immunosuppression and certain cancers. First described over 55 years ago, FeLV has been the subject of intense research interest, which has led to increasingly robust diagnostic assays and efficacious vaccines. While the prevalence of this infection in domestic cats has reduced in many geographic regions, the disease is still something of an enigma and can spread quickly, particularly within naïve 'multi-cat' populations such as shelters and breeding catteries, as well as within pet homes with multiple cats. An important goal in order to reduce the prevalence further is understanding the FeLV status of every cat at risk of infection.