Concrete is a versatile and widely used building material, but it porous and the hydrated cement in it has a very high surface area. External agencies such as stains tend to soak into it on contact and may react with the hydrated cement. As a result, removal of the stain often becomes difficult and requires special treatment. This Digest will discuss forms of stain that may appear on concrete surfaces after a building has been in use, stains resulting from accidental spillage of materials such as oil, blood, ink, beverages, asphalt, or from natural exposures such as corrosion, growth of microorganisms, etc. It will not deal with protective coatings applied to concrete surfaces as a preventive measure against surface attack, and is meant only as a guide to measures available for removing the more common forms of unwanted stain.