The aim of this review is to present information about leukemia caused by feline leukemia virus (FeLV) infection in felids. Despite widespread vaccinations, leukemia is still one of the most common diseases in cats. FeLV infection occurs by direct contact between animals, especially through contact with saliva. Infection with this highly contagious virus is characterized by immunosuppression and non-specific signs, such as cytopenia or tumorigenesis. The virus is transmitted horizontally through close contact between cats and vertically from infected queens to their kittens. Persistently infected cats are the reservoir of the virus. Rapid ELISA tests and immunochromatography play a major role in diagnosis, but RT-PCR is the most reliable diagnostic method. There is no specific therapy against FeLV, so treatment is mainly symptomatic. Research on the use of antiviral drugs, i.e. tenofovir, raltegravir, and fozyvudine, is underway. Proper prophylaxis, including regular vaccination of animals, is important to minimize the risk and consequences of infection