Current concepts on the mechanisms of urolithiasis have been briefly discussed. Some of the various factors involved are schematised in fig. 8. It seems likely that both the formation of a calcifiable matrix and urinary supersaturation with respect to the depositing salt are required for a calculus to grow. Emphasis has been laid furthermore on the role of crystallization inhibitors especially pyrophosphate, which protect normal urine from stone formation. This latter concept opens new aspects not only in the understanding of the pathogenesis of stone disease, but also in its treatment.