Fatigue strength under variable stress amplitude is a useful method in lightweight design. It allows for reliable designing and dimensioning of structural components under realistic environmental conditions. Special emphasis is put on fatigue stress. For designing and dimensioning, the expected stress (load) is compared with the capacity to withstand loads (fatigue strength). In this process, the safety necessary for each particular application must be ensured. A prerequisite for ensuring the required fatigue strength is a reliable load assumption.The authors describe the transformation of the stress-time functions which have been measured under operational conditions to spectra or matrices with the application of counting methods. The aspects which must be considered for ensuring a reliable load assumption for designing and dimensioning are discussed in detail. Furthermore, the theoretical background for estimating the fatigue life of structural components is explained, and the procedures are discussed for numerous applications in practice. One of the prime intentions of the authors is to provide recommendations which can be implemented in practical applications.