Fibres are a very important form of reinforcement. They have the potential to provide the greatest level of strengthening to the design of a composite material.

There are three principal types of fibres: very thin single crystals called whiskers, small diameter polymer or ceramic fibres, and thicker wires that are usually made of metal.

The most common are the small diameter fibres. Glass and carbon fibres are the principal fibres used in fibre reinforced composites although other polymeric fibres, such as aramid fibres, are also used.

Glass is by far the most widely used fibre for reinforcing and is the lowest in cost. Carbon and aramid fibres are more expensive but are used in many high technology applications, such as aerospace, due to their high strengths and low densities.