Human herpesvirus 8 (HHV8), or Kaposi’s sarcoma-associated herpesvirus (KSHV), is the most recently discovered human tumour virus and involved in the pathogenesis of Kaposi’s sarcoma, primary effusion lymphoma and the plasma cell variant of Multicentric Castleman’s Disease. It is non-pathogenic in the majority of otherwise healthy individuals but highly oncogenic in the context of HIV-1 infection and iatrogenic immune suppression. Other co-factors may be involved in the pathogenesis of KS in HIV-uninfected and not immunosuppressed individuals, e.g. in African endemic KS, but none has so far been identified. Four major clades of HHV8, defined on the basis of variability in the K1 gene, have so far been found, of which two are largely confined to Africa and Southeast Asia, respectively. Two highly divergent variants of the K15 gene also exist, but at present there does not appear to be a strong link of any of these variants with particular pathologies. KSHV establishes a latent infection in the majority of KS spindle (endothelial tumour) cells, and in the neoplastic B cells of primary effusion lymphoma. The HHV8 genome contains several homologues of cellular genes known to regulate cell growth and differentiation, as well as a gene with similarities to another viral oncogene. Although some of these have been shown to have transforming properties in vitro, their precise role in oncogenesis is still under investigation.