AbstractPolyomavirus BK (BKV) has ebeen identified as the main cause of polyomavirus‐associated nephropathy, a major cause of renal allograft failure. Although BKV‐associated nephropathy develops in only 2% to 5% of renal transplant recipients, its prognosis when present is very poor, with irreversible graft failure developing in 45% of affected patients. While the use of urine cytology for the detection of decoy cells has been in use for decades, other diagnostic modalities to detect BKV have emerged, including tissue biopsy, polymerase chain reaction, viral culture, and serology. Currently, there is no consensus regarding the laboratory technique best suited for clinical monitoring. This review article will discuss essential and clinical features of polyomavirus, followed by a discussion pertaining to the various diagnostic modalities that contribute to detecting polyomavirus‐associated nephropathy. Diagn. Cytopathol. 2009. © 2009 Wiley‐Liss, Inc.