Axles and shafts are vital parts of a large number of machines. Therefore, their function and features are described at the begining of the paper. Many of their fractures, as well as the fractures of other metal structures and machine parts, occured at stresses much lover than the yield strenght of the material, witch indicates the fatigue of material. The strenght of material, static and dinamic, and the method of describing the resistence of structures or machine parts to fatigue damage of a Wöhler curve and Smith diagram are described in detail. Influencing factors on the strenght of machine parts and the way of dimensioning axles and shafts are explained. Moreover, several axle and shaft fractures that occurred in practice were processed in this paper. The ways and places where fracuters of axles and shafts most often occur are analyzed, and variants of geometric design of fractures sites that could have prevented their occurrence are presented. By correctly making the geometry of the axle and shaft transition, we can extend their service life from finite to infinite service life. At the end of the paper, with help of computer simulations in the Autodesk Inventor softver on a 3D model, the stress concentration factors for axles and shafts for axial load, bending and torsion were obtained. Of which the axial load has the greatest influence on the stress concentration factor, and the least on the torsion