This book is an attempt to provide a unified methodology to derive models for fatigue life. This includes S-N,—-N and crack propagation models. This is not a conventional book aimed at describing the fatigue fundamentals, but rather a book in which the basic models of the three main fatigue approaches, the stress-based, the strain-based and the fracture mechanics approaches, are contemplated from a novel and integrated point of view. On the other hand, as an alternative to the preferential attention paid to deterministic models based on the physical, phenomenological and empirical description of fatigue, their probabilistic nature is emphasized in this book, in which shastic fatigue and crack growth models are presented. This book is the result of a long period of close collaborationbetween its two authors who, although of different backgrounds, mathematical and mechanical, both have a strong sense of engineering with respect to the fatigue problem. When the authors of this book first approached the fatigue field in 1982 (twenty six years ago), they found the following scenario: 1. Linear, bilinear or trilinear models were frequently proposed by relevant laboratories and academic centers to reproduce the W¨ ohlerfield. Thiswas the case of well known institutions, which justified these models based on clientrequirementsorpreferences. This led to the inclusion of such models and methods as, for example, the up-and-down, in standards and official practical directives (ASTM, Euronorm, etc.), which have proved to be unfortunate.