Marek's disease (MD) and lymphoid leukosis (LL) are two distinct viral diseases that cause tumor mortality in chickens. Marek's disease, being horizontally transmitted, is controlled through biosecurity measures and vaccination. Prevention of early exposure before vaccine immunity is established is most important. Some multi-house growing farms have converted to all single-age placements to break the ongoing cycle of transmission. Vaccination against MD involves either in ovo or day-old administration of live vaccine, including single or multiple serotype products. Field viruses appear to adapt over time and become resistant to the prevalent vaccine. The Rispens vaccine (CVI-988) has shown good efficacy against recently emerging very virulent MD strains in the U.S. Genetic resistance of the host to MD and control of other immunosuppressive diseases also affects MD susceptibility. Lymphoid leukosis is primarily vertically transmitted and therefore controlled by elimination of shedder hens at the primary breeder level. Depending upon the genetic type, commercial performance of laying hens may be greatly improved by eradication of the LL virus from the breeding stock.