There are six basic mechanisms of failure: failure due to excessive plastic deformation as the result of static overload or impact, instability, creep, stress corrosion, fatigue and brittle fracture. Conventional design methods almost entirely eliminate the risk from the first two causes, and to a very large extent the risk of failure from creep. Fatigue is the most common cause of failure, and brittle fracture the most spectacular. In the occurrences of failure, joints and in particular the presence of welded joints, frequently play a decisive part owing to: (a) the stress concentration they produce, (b) the residual stress caused by welding, and (c) the metallurgical changes produced by welding. The remedy for avoiding these failures lies in two directions: the wider spread of what is already known mainly through normal educational channels, and an intensification of the research effort in those areas where knowledge is still fragmentary.