Urinary protein macromolecules have long been thought to play a role in influencing the various phases of urolithiasis including nucleation, growth, aggregation of mineral crystals and their subsequent adhesion to the renal epithelial cells. However, compelling evidence regarding their precise role was lacking, due partly to the fact that most prior studies were done in vitro and results were highly variable depending on the experimental conditions. The advent of genetic engineering technology has made it possible to study urinary protein macromolecules within an in vivo biological system. Indeed, recent studies have begun to shed light on the net effects of loss of one or more macromolecules on the earliest steps of urolithiasis. This paper focuses on the in vivo consequences of inactivating Tamm-Horsfall protein and/or osteopontin, two major urinary glycoproteins, using the knockout approach. The renal phenotypes of both single and double knockout mice under spontaneous or hyperoxaluric conditions will be described. The functional significance of the urinary macromolecules as critical defense factors against renal calcification will also be discussed.