The application of ultrasonic guided waves for the long range inspection of large structures is reviewed. The technique is in routine commercial use on pipes and this technology is described. However, the method is much more difficult to apply on complex structures where the reflections from multiple features are not resolved in time. Here it is necessary to look for changes relative to a baseline condition as part of a structural health monitoring strategy; this requires a method to compensate for benign changes such as temperature variations, and possible methods for achieving this are discussed
