Wood-plastic composite (WPC) lumber has been marketed as a low-maintenance, high-durability product. Retail sales in the United States were slightly less than $1 billion in 2008. Applications include docking, railing, windows, doors, fencing, siding, moldings, landscape timbers, car interior parts, and furniture. The majority of these products are used outdoors and thus are exposed to moisture, decay, mold, and weathering. WPCs are composites made primarily from wood or cellulose-based materials and plastic(s). The wood utilized is usually in particulate form, such as wood flour or very short wood fibers. Because of the thermal stability of wood only thermoplastics that melt at temperatures equal to or below 200°C (392°F), which is the degradation point of wood, are utilized in the production of WPCs. The most common plastics used for WPCs are polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC). The WPC blends can vary in percentage, with wood content up to 70%, but 50% wood/50% plastic is common. It was first thought that mixing plastic and wood together would result in plastic encapsulation of wood, thereby preventing both moisture sorption and fungal decay. However, after over a decade of use in outdoor exposure, issues have surfaced that are caused by wood decay, susceptibility to mold, and polymer degradation. After the first generation products showed these problems, the industry began io address the performance issues by improving the formulations with additives, including fungicides, mildewcides, and photostabilizers. Further improvements are continually being investigated today. Researchers around the globe are currently working on understanding the fundamental mechanisms of degradation and on ways to improve WPCs.