Fillers not only reduce the cost of composites, but also frequently impart performance improvements that might not otherwise be achieved by the reinforcement and resin ingredients alone. Fillers are often referred to as extenders. In comparison to resins and reinforcements, fillers are the least expensive of the major ingredients. Fillers can improve mechanical properties including fire and smoke performance by reducing organic content in composite laminates. Also, filled resins shrink less than unfilled resins, thereby improving the dimensional control of molded parts. Important properties, including water resistance, weathering, surface smoothness, stiffness, dimensional stability and temperature resistance, can all be improved through the proper use of fillers.

Use of inorganic fillers in composites is increasing. When used in composite laminates, inorganic fillers can account for 40 to 65% by weight.