Composites are formed from two or more dissimilar materials, each of which contributes to the final properties. Unlike metal alloys, the materials in a composite remain distinct from each other at the macroscopic level (unless nano composites are involved).

Most engineering composites consist of two materials: a reinforcement and a matrix. The reinforcement provides stiffness and strength; the matrix holds the material together and serves to transfer load among the discontinuous reinforcements. The most common reinforcements are continuous fibres, either straight or woven, short chopped fibers and particulates. The most common matrices are various plastic resins.

Metal and other traditional engineering materials are uniform, or isotropic, in nature. This means that material properties, such as strength, stiffness, and thermal conductivity, are independent of both position and the choice of coordinate system. The discontinuous nature of composite reinforcements, though, means that material properties can vary with both postion and direction. For example, an epoxy resin reinforced with continuous graphite fibers will have very high strength and stiffness in the direction of the fibres, but very low properties normal or tansverse to the fibers.

This directionality increases the complexity of structural analysis. Isotropic materials are fully defined by two engineering constants: Young's modulus, E, and Poisson's ratio v. A single ply of a composite material, however, requires four constants, defined with respect to the ply coordinate system. The constants are two Young's moduli (the longitudinal modulus in the direction of the fibers, and the transverse modulus normal to the fibers, one Poisson's ratio v12, called the major Poisson's rato and one shear modulus). A fifth constant, the minor Poisson's ratio v21, is determined from the other properties.

The longitudinal modulus is largely a function of the fibre modulus, whereas the transverse and shear moduli are largely functions of the resin or thermoplastic matrix modulus. Thus, higher modulus fibres will raise the longitudinal modulus of the composite, but will have negligible effect on the other properties. As with the elastic properties, the strength is significantly greater in the longitudinal than in the transverse direction. Also note that compressive strengths are significantly lower than tensile strenghts. This difference is accounted for when we analyze composite structure for failure.