Through decades and centuries of indoor and outdoor exposure, objects become soiled with atmospheric pollutants, particulates, organic matter, and soluble salts. Chemical reactions cause surface scales, and the loss of materials. In general, particularly for aged, friable surfaces, there are no completely safe cleaning measures. Most methods have some surface impact such as micro abrasion, loss of loosely bonded substrate, and possible color change. Successful cleaning programs improve surface appearances and limit harm by designing treatments with sensitivity to object fragility, object material composition, existing soiling compositions, environmental conditions, need for future treatment, and anticipated maintenance cycles. Pretesting in discrete locations is essential, particularly for large-scale architectural efforts.