Pipelines are considered the safest and most economic way of transporting gas or liquids over long distances. Material degradation such as corrosion or cracking, however, can lead to premature failure with potentially catastrophic impact on man and environment. On the other hand, many pipelines are still in sound condition at the end of their design life thus enabling safe future operation. In any case non-destructive inspection techniques for the detection and sizing of material damage are required, if the integrity of pipelines is to be reliably assessed. This is achieved by means of so-called intelligent pigs which allow to inspect up to several hundred pipeline kilometers in one run with respect to special damage types such as e.g. corrosion damage. For the important issue of crack detection, however, intelligent pigs have in the past not been available as the electronics and data processing components required for this application were not at hand before. With the tool referred to in this article, this gap has successfully been closed. The results obtained from inspection of almost two thousand kilometers of operational pipelines (crude oil and gas) confirm that the reported crack detection tool meets the requirements for reliable in-line crack inspection. The new tool can detect axial crack-like defects With lengths> 30 mm and depths > 1 mm in the base material as well as in the weld area with high reliability. Offline recognition and classification of the recorded ultrasonic indications is ensured by the multiple detection concept which allows for confident discrimination of the variety of reflector types encountered in pipeline steels. Based in particular on the positive results of extensive veri fication work, the use of ultrasonic inline inspection (crack detection, wall thickness measurement) has been approved by the German TÜV as a means of pipeline integrity testing.