In the formation of calculi within the urinary tract, two fundamental factors are critical when crystals precipitate and stones form: They are solute and the solution itself, which must be oversaturated if crystals are to form and to grow. Several elements present in the urine influence solutions and solute and, thus, stone formation. Because all forms of medical therapy influence these elements favorably to prevent stone formation, this update reviews why stones form and how specific medical therapy works to prevent their formation.