Herpesviruses are present in most species throughout the animal kingdom and are classified into three subfamilies, α, β and γ, on the basis of their biological properties and genome sequences. A striking feature that is shared by many of the γ-herpesviruses is their ability to induce neoplastic disease in the host. This review focuses on three γ-herpesviruses: Epstein–Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV) and herpesvirus saimiri (HVS), and discusses the diverse array of EBV, KSHV and HVS viral genes that are involved in transformation, cell signalling, episomal maintenance and cell proliferation.