A knowledge of the composition of urinary stone is important because urolithiasis is a recurrent disease in many people and preventive measures must be based on such information. In stones of mixed composition knowledge of the structure, as well as of the composition, may be important in delineating pathogenesis. Analysis by the crystallographic technics of polarization microscopy and x-ray diffraction photography permit accurate identification of all parts of the stone, including a very small nucleus, and are preferable to chemical methods. The crystalline compounds in urinary calculi and the details and significance of calculus structure are described. © 1968.