These research activities focus on the investigation of the microstructure, the mechanical properties and the rolling contact fatigue properties of the bearing steel AISI M50 (80MoCrV42-16). This through-hardening steel comprises MC and M2C carbides and is favourably used to produce roller bearings of aircraft turbines, in which rolling contact fatigue occurs because of the high cyclic loading due to over-rolling. The principal aim of the work was to investigate the relationship between material parameters and the rolling contact fatigue behaviour to describe the crack initiation and damage phenomena until pitting occurs in order to evaluate the optimization potential for an enhancement of the rolling contact fatigue properties. Investigations using metallographic methods like optical and scanning electron microscopy with EDS and EBSD, TEM, XRD and electron microprobe were made as well as measurements of the mechanical properties by means of tensile, compres-sion, fracture toughness and LCF tests. Only minor differences in the content and type of non-metallic inclusions, the segregations and the structure and size of carbides were found, but they obviously did not cause differences in the common mechanical properties. In order to study the rolling contact fatigue properties, a large number of ball-on-rod-tests were performed until pitting occurred or loading was done with pre-defined number of cycles. The experiments focussed on the characterization of the number of cycles until pitting occurs, in order to determine the rankings of the investigated alloys with respect to the achieved lifetime on the one hand, and on experiments to investigate damage initiation and progress on the other. Rolling contact fatigue loading leads to widespread microstructural changes such as the so called “white-etching-areas” in a certain region below the raceway, and to very localised ones, the called “butterfly-wings”, next to inhomogeneities. „White-Etching-Areas” are regions with high cyclic strain in a certain depth, depending mainly of loading and geometry, where the cyclic plastification leads to a recrystallisation and in some instances to an increase of hardness. „Butterfly-Wings“ are distinct areas with grain size in the nanometer range, where sometimes microcracks close to the boundary to the matrix exist. Also in this case a recrystallization due to the high cyclic plastifiction occurs. In this ultrapure steel primarily carbides or carbide agglomerations are the root cause for the strain accumulation. Finally, a comparison between specimens from the ball-on-rod-tests and from tests at a complex bearing test rig with loading conditions similar to that in real bearings was performed. Despite the different loading conditions in the two tests, the damage phenomena are very similar. The main pitting initiation sites for pitting in this ultrapure steel are mainly carbides or carbide agglomerations instead of non-metallic inclusions.