Fatigue behavior of composite materials and structures is a complex phenomenon consisting of cyclic load-induced events and processes over time which determine long-term performance. The events and processes, generically called damage, combine in such a way as to alter the state of the material and change the response of the composite to the extent that it may fail to satisfy its intended service requirements. This chapter begins with an illustration of some of the distinctive features of fatigue behavior of composites, followed by a description of the fatigue damage process. The mechanics of stress redistribution due to damage are presented in the context of experimental evidence. Finally, fatigue behavior of several composite material systems is discussed in terms of strength, stiffness, and life of notched and unnotched composite laminates under various loading modes.