Composites are solid heterogeneous materials consisting of two or more phases. However, many composites of practical interest are composed of just two phases, namely, dispersed phase and continuous phase. Interest in particulate composites has expanded as these two-phase mixtures often provide an advantageous blend of properties of the individual materials. The solid propellants commonly used in aerospace propulsion are particulate composites consisting of particles of solid oxidizer, typically ammonium perchlorate, dispersed in a matrix of plastic fuel such as polybutadiene. The most common advanced composites are based on a polymer matrix. However, there are other important particulate composites based on a nonpolymeric matrix such as metal matrix composites and ceramic matrix composites. Composites with pores find many engineering applications ranging from acoustic absorption to heat shields in reentry vehicles. A composite material composed of isotropic dispersed phase distributed randomly in an isotropic matrix has isotropic effective properties.