Marek's disease (MD) vaccines have been in almost universal use since the early 1970's and constitute the first effective practical means for the control of any neoplastic disease in man or animals. Five types have been described: 1) attenuated variants of oncogenic strains (Serotype 1) of Marek's disease virus (MDV) produced by serial passage in cultured cells; 2) naturally nononcogenic MDV strains (Serotype 2); 3) nononcogenic herpesvirus from turkeys (HVT, Serotype 3); 4) inactivated cells or cell membrane components from infected cell cultures containing viral-associated antigens; 5) inactivated cells or membrane components from lymphoblastoid MD tumor cell lines containing presumed tumor-associated antigens. HVT vaccines, in either cell-associated or cell-free form, are most frequently used for vaccination of commercial flocks. Both cell-mediated and humoral immune responses are mounted and each may contribute to vaccinal immunity. Probably the most significant responses are against the early (cytolytic) phase of MDV infection, although vaccines apparently also induce a response against tumor-specific antigens as well.