The deterioration of stone and other brittle building materials is determined by mechanical (thermal shock, load, human action, etc.), physical (salt crystallization, frost), and chemical (acids in the air) factors. The rate of deterioration strongly depends upon the memory (salts, cracks) a material carries along of previous abuse by man or the environment. It is extremely difficult in such conditions to assess deterioration rates and to correlate them with single deterioration factors (e.g., an acid pollutant in the air). Conservation practices may aim at improving the environmental conditions of exposure of the material; this may include a generalized action to reduce pollution but also a review and maintenance of building features which control the action of water and reduce thermal excursion. Technological processes directly applied to the materials (including a series of cleaning, consolidation, and protection treatments) are unable to insure long-range preservation, even if great progress were realized in the last years, unless they are followed by routine maintenance operations.