Exposed stone and masonry deteriorate in a variety of ways, depending upon the chemical reactivity, grain size and texture, porosity, and degree of induration SEM, employed in conjunction with x-ray diffraction, optical microscopy, and chemistry, discloses the internal structure and texture of the stone, as well as the presence of adventitious substances and hence is effective in elucidating the susceptibility to, and state of, decay and its causes in specific cases. Limestones and marbles show significant direct solubility in environmental water, as well as enhanced solubility due to chemical reaction with air pollutants. Water-soluble materials either produced in the stone or carried into it from adjacent sources deposit in cracks and pores at the surface as the stone dries and generate pressures that break down intergranular attachments and cause the stone to crumble, crack and exfoliate. Quartzite and arkosic sandstones deteriorate principally under the influence of the stresses accompanying freeze-thaw and wet-to-dry cycling. Slates suffer delamination; zeolitic stone is subject in addition to specific effects of a chemical nature. Synthetic resin-powdered stone composites used as a replacement for decayed natural stone tend to fail because of the crystallization effects that occur along shrinkage crevices in the resin layer. The micrographs presented show the application of SEM in characterizing the type of stone present and its condition in specific monuments, as well as in identifying the principal mecha-nism responsible for the manifestations of decay.