The accurate modeling of the fatigue behavior of composite materials under constant amplitude (CA) cyclic stress is in itself a difficult task but when loading is of variable amplitude (VA), the situation is even more complicated since an appropriate damage metric and rules for its summation must be defined. One of the widely used approaches is based on the theoretical formulation and use of a damage summation rule to predict life under VA loading without recourse to experimental observation of the damage accumulation process. The most popular and best-known example of this category, which does not always lead to accurate results however, is the linear Palmgren-Miner rule. Other summation rules were also proposed as alternatives to the use of Palmgren-Miner rule to accurately predict the fatigue lifetime of glass-fiber reinforced plastic (GFRP) and CFRP composites loaded under block or VA loading patterns. An alternative to this classic fatigue life prediction methodology is the residual strength fatigue theories, where residual strength is used as the damage metric. Comparison of the remaining strength of the material to the static strength allows the estimation of the fatigue cycles until failure. The basic fatigue modeling introduced in previous chapters of this book for the interpretation of the fatigue data (Chapter 2), residual strength theories (Chapter 3), and constant life diagrams (Chapter 6) is combined here to establish fatigue life prediction methodologies.