Metal matrix composites (MMCs) are engineered materials comprising a metallic matrix reinforced with ceramic, metallic, or metastable phases to achieve superior mechanical, thermal, and wear properties. Common matrices include lightweight metals such as aluminum, magnesium, and titanium, as well as steel and nickel-based superalloys for high-temperature applications. Reinforcements range from continuous fibers to particulates and nanoscale phases, each imparting distinct performance benefits. Classification into laminate, fiber-reinforced, particle-reinforced, and unconventional composites reflects their diverse architectures and property enhancements. Strengthening mechanisms—including load transfer, thermal mismatch, dislocation density increase, Orowan strengthening, and grain refinement—contribute to their high performance. MMCs are widely used in aerospace, automotive, electronics, and sports industries where strength-to-weight ratio, thermal stability, and wear resistance are critical. This chapter reviews their classification, properties, and strengthening mechanisms, providing insights into their growing role in advanced engineering applications.