While plastics have many desirable properties, such as the ease with which they can be used to make complex parts, their relatively low stiffness and strength can limit their structural applications. However, this limitation can be overcome by forming composites in which the plastic matrix acts as a glue that holds stiffer and stronger fibers together. This chapter discusses several stiffening mechanisms, including the use of aligned fibers in advanced composites, and small chopped fibers, flakes, and particulates in injection moldable compounds. Characteristics of the commonly used fibers, flakes, and particulates are also described.