ESDU 91027 discusses the probabilistic nature of non-destructive examination (NDE) to search for defects such as manufacturing flaws (for example, inclusions, porosity, weld defects) or cracks generated in service. This leads to the selection of an acceptable defect size that gives a balance between the probabilities of rejecting satisfactory components and of accepting unsatisfactory ones. The different philosophies applying in NDE during manufacture or in service are considered. The importance is emphasised of taking into account the requirements of NDE at the design stage to ensure that components and assemblies can be examined economically and effectively both during manufacture and in service. Guidance is provided on the choice of suitable NDE procedures for a specific application by tabulating the mechanics of the various methods, their typical applications, advantages and limitations. The approximate defect size likely to be found by each method is stated. Methods discussed are direct or indirect visual examination, endoscopy, liquid penetrant, magnetic particle, eddy current, ultrasonic, radiography, magnetic, acoustic or vibration methods. The use of stress jacking to improve detection and proof testing is also considered.