An introduction to the fatigue properties of Fiber Reinforced Plastic (FRP) composites is given. The implications of these properties in the design of composite structures are stressed. Topics covered include: an overview of FRP composites, a definition of fatigue and fatigue testing, a comparison of typical metal and FRP fatigue mechanisms, and a discussion of the major fatigue design considerations for FRP composites. Finally, a system’s view is presented to illustrate the inter-dependence of material, design, and processing considerations in fatigue of FRP composites.