The majority of all service failures can usually be traced to deformation and fracture under cyclic loading including fretting fatigue, corrosion fatigue, impact fatigue, creep fatigue and so on. As a result, a large volume of literature on fatigue fracture has appeared. There is no doubt that metal matrix composites suffer from the various forms of fatigue damage. However, the fatigue behaviour of metal matrix composites is usually thought to be superior to that of the corresponding unreinforced matrices, especially in longfibre reinforced metal matrix composite systems. Most importantly, we must investigate each fatigue process: cyclic softening and hardening, fatigue crack initiation, low-cycle fatigue, high-cycle fatigue and final rapid fracture, in order to attempt to design microstructures of the metal matrix composite for optimum fatigue resistance. In this chapter, each fatigue stage for discontinuously reinforced aluminium matrix composites will be reviewed and compared with unreinforced alloys. Static loading behaviour is discussed in chapter 12.