The endurance limit and the mechanisms of fatigue crack initiation in the high cycle regime were investigated using round specimens of the bearing steel 52100 under longitudinal forces and torsional moments and combinations of these loads. Three specimen types were examined: smooth specimens and specimens with circumferential notches with radii of 1.0 and 0.2 mm. The influence of mean and multiaxial stresses on the endurance limit can be understood by consideration of crack initiation mechanisms and micromechanics. Crack initiation took place at oxides, carbonitrides and at the surface. The mechanisms of crack initiation could be related to the load type: Loads with rotating principal stresses are more damaging for nitrides than for oxides. Increasing maximum stresses are more dangerous for nitrides than for oxides, and introduce more damage to the surface than to the nitrides. Normal stresses are more damaging for oxides than shear stresses. The endurance limits were calculated by means of an extended weakest-link model which combines volume and surface crack initiation with related fatigue criteria. For volume crack initiation the criterion of Dang Van was used. For the correct description of the competing surface crack initiation, a new criterion was applied. With this concept, a prediction of the endurance limit is possible for loads which produce critical planes and range within a limited regime of stress ratios.