This chapter familiarizes readers with the methods used to quantify the effects of fatigue on component lifetime and failure. It discusses the development and use of S-N (stress amplitude vs. cycles to failure) curves, the emergence of strain-based approaches to fatigue analysis, and important refinements and modifications. It demonstrates the use of approximate equations, including the method of universal slopes and the four-point correlation technique, which provides reasonable estimates of elastic and plastic lines from information obtained in standard tensile tests. It also discusses high-cycle, low-cycle, and ultra-high cycle fatigue and presents several models that are useful for fatigue life predictions.