JC virus (JCV) and BK virus (BKV) are human Polyomaviruses of the papovavirus family, which also includes a simian vacuolating virus 40 (SV40). Human Polyomaviruses were first isolated in 1971 from the brain (JCV) and urine (BKV) of two different patients. Human Polyomaviruses have a limited and specific tissue tropism infecting the renal tubular cells, the urothelium, the B cells and the brain cells. The virus infects the majority of the human population with seroconversion occurring during adolescence. The detection of the virus may be cytological, pathological, virological or immunological. Following a typically subclinical primary infection, Polyomavirus establishes a life-long persistent infection, especially in the urinary tract. BKV is known to reactivate and cause severe disease in immunosuppressed patients. The presence of Polyomavirus outside conditions of immunosuppression raises the question of its meaning and its therapeutic management. Given the ubiquitous nature of the virus and its strong association with cancer in animal models, they may play an etiological role in human malignancies. Here, we describe the biology of human Polyomaviruses, review their non-malignant and malignant potentials, and discuss the therapeutic aspect.