Infection with BK virus (BKV), a member of the Polyomavirus (PV) family, is ubiquitous, with the virus remaining in a latent form in the kidney and urinary tract. This infection is usually asymptomatic, but with impairment of the cellular immune system the virus can reactivate and lead to tissue damage. In recipients of bone marrow and solid organ transplants, PV reactivation can be associated with disease in urinary tract and kidneys. BKV was first discovered in 1971 from the urine of a kidney transplant recipient who had developed ureteral stenosis 4 months after transplantation. While much of the subsequent research focuses on patients after renal transplantation, we will review PV impact in patients after bone marrow transplant (BMT) and those with non-renal solid organ transplants.