Crack initiation and early crack propagation are decisive elements of material fatigue in metallic materials, and the behavior on the microscale level in this phase greatly influences the resulting lifetime of a given material, and subsequently, its usefulness and attractiveness for specific applications. It is impossible to quantify this key phase with non-destructive testing such as ultrasonic inspection, nor are the common methods of elastic and elastic-plastic fracture mechanics applicable. Therefore, prediction possibilities and purposeful microstructure design towards improved material behavior are highly desirable. Part one of this book is a comprehensive overview on the history of fatigue and fracture research and the basic concepts of fatigue-life prediction, fracture mechanics, experimental methods, and the deformation behavior of metallic materials. The second part gives an overview on experimental studies and theoretical approaches on the interactions between the material's microstructure, the mechanical loading conditions, and the corresponding fatigue crack propagation behavior. Thirdly, the experimental results and hypotheses presented are discussed by means of phenomenological and physical models the latter being based on the numerical boundary element method. These models are intended to be applied to future mechanism-oriented life prediction of technical materials under complex service conditions.