The service life of a gearbox is determined by many factors. The bearings in the gearbox play a major role since they themselves deliver an important function@ and in addition interact with the shafts@ the casing and the oil. Without a doubt@ the sizing of the bearings is of great importance for the gearbox reliability. Since more than 50 years the bearing dynamic carrying capacity has been used to determine a suitable size needed to deliver a sufficient fatigue life - but despite the advanced calculation methods developed@ the methods do not fully predict service life. Producers of high quality bearings have introduced high performance class bearings and@ lacking better ways to express the improved performance@ this is only represented by increased dynamic carrying capacity. The availability of high-strength shaft materials in combination with bearings with high carrying capacity allows slimmer shafts to be used. The modulus of elasticity remains the same@ so seat design for bearings and gears must be given close attention. This paper covers the following: sizing of bearings based on dynamic carrying capacity and how this relates to service life; how the design of the interface between bearing and shafts should be adapted to modern shaft materials; how the design of the interface between bearing and gearbox casing influences service life of the gearbox; and influence of modern electric motor speed controls in bearing type selection.