BK virus infection after kidney transplantation has been a subject of great interest in the past decade. This article traces the discovery of BK virus and the subsequent development of our knowledge about this emerging pathogen. The pathobiology of the virus is summarized with particular reference to epidemiology, interactions with host cell receptors, cell entry, cytoplasmic trafficking and targeting of the viral genome to the nucleus. This is followed by a discussion of clinical features, laboratory monitoring and therapeutic strategies. Finally, we present potential cellular mechanisms that explain the basis of virus-mediated damage to the human kidney.