The Art Conservation Research Foundation, Ltd. Annual Report, 1988, records the progress made during the year by the investigations on the causes of decay of surfaces exposed to environmental changes. Aimed at lay readers the report explains the model of the mechanism of decay of outer surfaces (skins) of materials, as discovered by this research. The importance of the old rule of Fat Over Lean as a general condition for the preservation of outer surfaces is explained as follows: A surface can only crack or blister if it is harder than the substrate which carries it. A schematic illustration of stress valves, without using the term, shows their effect on six practical examples drawn from paintings and other surfaces found in nature. -- AATA