In animal studies, polyoma viruses have been found to be viral agents for oncogenesis and to produce a wide range of pathological lesions in experimental animals, including a variety of neoplastic tumors. The human polyoma viruses (JCV and BKV), along with their simian cousin (SV40), are ubiquitous viruses that are primarily associated with progressive multifocal leukoencephalolopathy (PML) and hemorrhagic cystitis, respectively, under specific conditions in immunocompromized individuals. Currently, polyoma viruses are now undergoing increasing scrutiny as possible causes for several human cancers. Evidence has been mounting recently that JCV, BKV as well as SV40 are potential oncogenic viruses in humans as well.