Composite materials are materials that are produced by combining two or more constituent materials with notably dissimilar chemical or physical properties. The individual elements remain separate and distinct within the finished structure, distinguishing composites from mixtures and solid solutions. The two main categories of constituent materials are the matrix (binder) and the reinforcement, with a portion of each kind needed at least. The reinforcement receives support from the matrix as the matrix surrounds the reinforcement and maintains its relative positions.

Composite materials offer significant benefits in various material performance aspects, exceeding the mono-material alternatives and particularly standalone use of the constituent parts. They merge two or more divergent materials to produce a result that displays the advantageous properties of the constituents. Typically, the materials are referred to as the matrix, which is the bonding agent, and reinforcement, which is generally a fiber component. The matrix materials are typically polymers, though they can also be ceramics and metals. The manufacturing methods of composite materials vary widely, and the key materials that are most commonly described as composites are glass, carbon, and Kevlar® fibers, bonded with resins.

Some examples of composite materials include reinforced concrete and masonry, composite wood such as plywood, and reinforced plastics such as fiber-reinforced polymer or fiberglass. Fiberglass was the first modern composite, developed in the late 1940s, and is still the most common composite material, making up about 65% of all the composites produced today. Composite materials are used in various applications, including boat hulls, surfboards, sporting goods, swimming pool linings, building panels, and car bodies.