The processes of stone deterioration are summarised. The role of silicones as water-repellent impregnant for stone is described. The advantages of a silicone impregnation of buildings are: high water-repellency, high gas permeability, prevention of moss growth, dirt-repellency. For the strengthening of stone silicic acid esters are recommended. Both strengthening and impregnation benefits may be achieved simultaneously by using a mixture of silicic acid and alkyltrialcoxy silanes. The importance of penetration is explained.