There has been a great interest in metal matrix composites during the last 10-15 years. The reason is their potential to achieve a high strength and good dimensional stability in a large temperature range and to obtain a good stiffness. The interest has particularly been focused on light metals reinforced with a continuous or discontinuous ceramic phase. The reinforcement and the matrix have very different properties such as thermal expansion and elastic modulus, which of course is the point in using this type of reinforcement. These differences between matrix and reinforcement gives rise to residual stresses after temperature changes or after plastic deformation.