The aim of this thesis is the verification of the technological effects of forging and heat treatment processes as well as of the influence of local multiaxial load conditions on the resulting fatigue behaviour. A systematic investigation of the individual process parameters of both forging and heat treatment processes indicates that the fatigue behaviour is markedly influenced by the local orientation of segregations as well as the local macro hardness. A statistically relevant influence of the local plastic strain and the grain size distribution on the fatigue behaviour could not be determined. The annealing process in the forging furnace causes a diffusion process in the barrier region of segregations resulting in a fatigue strength increase. Based on these results a simulation model for the calculation of synthetic S/N curves was derived. For the investigation of the influence of multiaxial loadings a new testing method was developed. The fatigue tests indicate that the influence of combined proportional loadings can be well described by several existing fatigue strength hypotheses. However, combined non-proportional loadings lead to some deviation of the calculated fatigue strength from the experimental results. Finally, two new strength hypotheses were proposed safety factor intensity hypothesis (SFIH) and damage intensity hypothesis (DIH) indicating good agreement with the fatigue tests of both quenched and tempered steels and steel cast alloys.