This book is written in memory of the late Prof. Mohan Narayanaswami Ranganathan, University of Tours, France, whose contributions to the fields of materials science, engineering, and fatigue have inspired generations of researchers. He had an unrelenting intellectual curiosity and a unique way of looking at things and drawing in knowledge from different areas to provide an entirely different perspective. When a given path seemed untraversable, he always managed to maneuver with his charm, charisma, and optimistic point of view. When Prof. Ranganathan passed away in 2022, he was in the process of writing this book. So, 13 of his colleagues and friends decided to complete it in his memory and honour his legacy by advancing the understanding of various fundamental aspects of fatigue and fracture in materials. The book is a collective effort of world-renowned experts, some of whom were mentored by Prof. Ranganathan in the early stages of their careers. It comprises 11 chapters that focus on in-service failure analysis and fundamental fatigue data, fundamental fatigue crack propagation analysis, fundamental multiaxial fatigue analysis, and the effect of environment, defect, fundamental physics, and very high cycle loadings on fatigue failure. The book integrates experimental insights with theoretical frameworks and is a valuable resource for researchers, engineers, and students engaged in materials science, mechanical engineering, fatigue, and related disciplines.