An integral review is given in this book on the fatigue phenomenon covering the fundamentals of fatigue damage initiation, relevant factors influencing fatigue crack propagation and fatigue life, random load analysis, and simulation for theoretical and experimental fatigue life assessment. The entire chain of problems related to fatigue of metals and structural components is covered. Specifically, it describes the low-cycle plastic properties and statistically interprets the material stress reaction, examining original results of investigations on inelastic deformations under high cycle cyclic loading and correlating them with a number of use parameters. The limit states of bodies with primary defects and their resistance to fatigue crack propagation are discussed. Measurements, analysis and real-time modelling of operating loads for experimental fatigue life verification are reviewed as well as introducing some new fatigue damage accumulation hypotheses based on dissipated energy. Various operating and environmental factors of the fatigue life are analyzed, including temperature, metal structures, corrosive environment, stress-strain amplitudes and their changes, random load (strain) properties, stress gradient frequency, mean level, etc. The work is intended for all those involved in research and development in the metal, machine and structure fields.