In historic structures, other criteria for evaluation of stone cleaning become as or more significant than mere cleanliness. Preservation of the original surface, the effect on the surface's future durability, environmental effects, and the nature and causes of distress, must be considered. New criteria must include not only these, but also the need to minimize any future intervention. A trained geologist can prove invaluable to the professional team in the evaluation of these criteria. The body of geologic knowledge contains the necessary information for evaluation. Geological evaluation can identify causes of distress, evaluate effects of cleaning processes on the materials of a structure, and identify methods that are most, or least, desirable. Methods can be equally applicable to the selection and evaluation of replacement materials. An illustration of the evaluation process is given using the Tribune Tower, Chicago, Illinois, and the Mormon Temple, Salt Lake City, Utah, as examples. Considerations for other materials are also described.