The term “wood-plastic composites” refers to any number of composites that contain wood (of any form) and either thermoset or thermoplastic polymers. Thermosets or thermoset polymers are plastics that, once cured, cannot be remelted by heating. These include cured resins, such as epoxies and phenolics, plastics with which the forest products industry is most familiar (see Chapter 10). Wood thermoset composites date to the early 1900s. An early commercial composite marketed under the trade name Bakelite was composed of phenol-formaldehyde and wood flour. Its first commercial use was reportedly as a gearshift knob for Rolls Royce in 1916. Thermoplastics are plastics that can be repeatedly melted, such as polyethylene, polypropylene, and polyvinyl chloride (PVC). Thermoplastics are used to make many diverse commercial products, such as milk jugs, grocery bags, and siding for homes. In contrast to the wood-thermoset composites, wood-thermoplastic composites have seen phenomenal growth in the United States in recent years. This chapter deals exclusively with wood-thermoplastic composites, which are now most often simply referred to as wood-plastic composites (WPCs) with the common understanding that the plastic always refers to a thermoplastic.