Loading time histories in machine elements most often have a complex character ensuing from the different purposes the machines are destined for and different operational conditions they work under. Vehicle elements undergo variable loading due to road roughness, ships due to interactions with sea waves, airplanes due to variable conditions during takeoff, flight and landing, pipes due to changes of temperature. This chapter discusses cyclic deformation, energy based fatigue criterion for uni- and multiaxial cyclic loading, cumulative fatigue damage hypothesis for cyclic block-type loading, and modelling of a crack growth rate. It presents a description of a fatigue crack propagation speed in terms of energy, making use of non-linear relationships within the field fatigue crack mechanics. The process of forecasting the fatigue life of machine elements requires except incorporating a fatigue life criterion the knowledge of laws ruling the development of a fatigue fracture appearing in these elements, that is, knowledge of the cumulative fatigue damage process.