Abstract: A method of monitoring a structure comprising the steps of:(a) transmitting a burst of acoustic energy through the structure from a transducer coupled to it,(b) detecting a received wave resulting from the burst at one or more transducers coupled to the structure, and(c) monitoring for any change in the structure after a given time (or event) by repeating the steps (a) and (b) with the coupling of the transducers being essentially unaltered, to detect a further received wave or waves; comparing the further received wave or waves with a corresponding reference received wave or waves; and determining for each comparison whether the received wave differs from the reference, any difference being indicative of the formation or growth of a defect in the overall acoustic path travelled by the acoustic energy.