- 2016 Elsevier Ltd. All rights reserved. This chapter reviews the damage mechanisms operating in composite materials under repeated application of loads. Microscopic observations of the failure events are presented and the progression of damage is deduced. The effects of fiber stiffness, matrix inelasticity, and the fiber-matrix interface on the damage mechanisms are discussed using a conceptual framework called fatigue life diagrams. Emphasis is placed on fatigue of unidirectional continuous-fiber composites with polymeric matrices.