This article discusses the types and causes of deterioration of stone. Mechanical, chemical, and biologically induced decay are reviewed, with an emphasis on the effects of polluted air. The author describes methods of monitoring the environment, such as hygrothermographs and internal probes, and simple methods of controlling conditions both inside and outdoors. Simple methods of dealing with surface deposits are briefly described. These deposits include surface salts, dust, greasy dirt, and biological residues on outdoor stone. The reader is advised against using coatings, metal tools or portland cement as a filler. White cementlime mortar is recommended. Other information includes how to protect stone from wet soil, and when excessive deterioration requires professional assistance. -- AATA