Fatigue is one of the most important failure modes of engineering components. The book presents recent research regarding the multiscale modelling of metallic materials during different stages of fatigue. The various parameters that are involved in each stage are investigated.Keywords: Fatigue Crack Initiation, Critical Resolved Shear Stress, Endurance Limit, Fatigue Crack Growth, Fatigue Lifetime Estimation, Multiscale Modelling, Paris Law, Tanaka-Mura Equation, Wöhler (S-N) Curve.