Metal matrix composites (MMCs) are engineered multiphase materials in which a continuous metallic matrix contains deliberately introduced reinforcing phases. Their properties arise from the combined effects of the matrix, reinforcement, interface, processing route and spatial architecture. The matrix provides metallic continuity, plastic deformation capacity, processability and thermal or electrical conduction, whereas the reinforcement is selected to modify stiffness, strength, hardness, wear resistance, thermal stability, corrosion response or functional behavior. From a practical interpretation standpoint, MMC performance should not be ascribed solely to the reinforcement fraction, but rather to the coupled effects of reinforcement distribution, interfacial bonding, porosity, residual stress, heat-treatment state, and architecture.