Composite materials include some of the most advanced engineering materials today. The addition of high strength fibers to a polymer matrix can greatly improve the mechanical properties, such as ultimate tensile strength, flexural modulus, and temperature resistance. The extent that the properties are enhanced will depend on the mechanical properties of both the fiber and matrix polymer, their volume relative to one another, and the fiber length and orientation within the matrix. This article will discuss how these various factors affect the improvement in properties. Among the factors which will be explicitly discussed are the stiffness of the reinforcing fiber, adhesion between the fiber and the polymer matrix and the overall dispersion of the high strength fiber in the polymer.