The choice of a particular (an appropriate) fatigue theory for composite materials and structures is based on the material’s behavior under the given loading pattern and the experience of the user. One of the most explicit and straightforward ways to represent experimental fatigue data is the S–N diagram that is preferred to other ways since it requires the minimum of experimental data. The effect of the loading is assessed by using constant life diagrams. This chapter aims to provide an overview of the commonly used S–N curves and the available constant life diagram formulations for the simulation of the fatigue behavior of composite materials and adhesively-bonded structural composite joints.