The overall objective of design against fatigue is to achieve adequate safety and reliability for minimum structural weight and, having this end in view, the fatigue performance of an aircraft structure is a major consideration in the design, testing and operation of an aircraft. This has not always been the case, indeed until the early 1950s the design of aircraft structures was based almost entirely on strength considerations with little regard to fatigue. However, in the decade following the Second World War a series of accidents both in the military and civil field, culminating in the Comet disasters, focused attention on the need to consider the problem of fatigue in design and provided the climate of opinion to support requirements for much more extensive and expensive full scale fatigue testing than had been contemplated hitherto. Since that time, advances in safety and reliability have been achieved in the face of customer requirements for major increases in overall fatigue life and for higher standards of day-to-day reliability of the structure. For example, in the military field a required fatigue life exceeding 6000 hours would not be exceptional now for a combat aircraft (Fig. 1) as compared with a life in service of about 1500 hours for such an aircraft in the years immediately following the Second World War. On the civil side, whereas lives of about 20 000 hours would have been considered adequate in the early 1950s, operators now require lives in excess of 60 000 hours. The European Airbus shown in Fig. 2 typifies such aircraft currently in service. Both military and civil aircraft are now expected to be in service for 20 or more years.