The presentation provides an introduction into the topic of the conference. The defects, the effect of which on the damage under cyclic loading is briefly discussed comprise non-metallic inclusions in steel and aluminum alloys, cavities such as pores, micro-shrinkages and un-welded regions in cast, sinter and additively manufactured alloys, graphite nodules, shrinkages and other items in modular cast iron, regions of defective microstructure, microcracks but also secondary notches such as undercuts and surface roughness. In order to cause the final failure of the structure, cracks have to initiate and to grow to a critical size. However, only a limited number of cracks really reaches this critical state. Most cracks arrest at some point during their propagation. The reasons for this range from microstructural barriers to mechanical mechanisms such as the gradual build-up of the crack closure phenomenon often in combination with notch effects. In other words: whether a crack arrests or not, depends on a range of factors including the size, the morphology, the proximity to surface and the clustering of the material defects. The underlying mechanisms will be briefly discussed in the presentation.