Weathering is the general term used to define the slow degradation of materials exposed to the weather. The degradation mechanism depends on the type of material, but the cause is a combination of factors found in nature: moisture, sunlight, heat/cold, chemicals, abrasion by windblown materials, and biological agents. Tall mountains weather by the complex and relentless action of these factors. All natural and man-made materials weather; for polymeric materials, the weathering rate is considerably faster than the degradation of mountains. Many of the materials we depend on for clothing and shelter undergo degradation by the weathering process. Wood is a material that has been used for countless centuries to provide people with shelter. Today we still depend on wood and wood-based products to provide this shelter. Our houses are usually made of wood, and the outermost barrier to the weather is often wood or a wood-based product (siding, windows, decks, roofs, etc.). If these wood products are to achieve a long service life, we must understand the weathering process and develop wood treatments to retard this degradation. Failure to recognize the effects of weathering can lead to catastrophic failure of wood products and other products used with wood. For example, if wood siding is left to weather for as little as one to two weeks before it is painted, the surface of the wood will degrade. During this short exposure period, the surface of the wood will not appear to have changed very much, but damage has occurred. Application of paint after one to two weeks of weathering will not give a durable coating. The surface of the wood has been degraded and it is not possible to form a good paint bond with the degraded surface. The paint will show signs of cracking and peeling within a few years. As the paint peels from the surface, the wood grain pattern can easily be seen on the back side of the paint. The peeling paint has lifted the damaged layer of wood from the sound wood underneath. The reasons for this will become apparent as we discuss the chemistry and degradation processes of wood weathering.