A monitoring system uses sound waves and/or ultrasound waves that are emitted into a structure being monitored from one or more monitoring devices and echo-waves from the interior and surface of the structure are received at one or more monitoring devices at locations of importance to the stability and operational worthiness of the structure. Received signals from echo-waves are processed so that operational noise is largely eliminated or reduced to an acceptable level then collected and stored or archived; newly received and processed signals are also compared with existing archive signals and a decision is made for each new signal if it is representative of the structure being unchanged or representative of the structure after significant structural change has occurred. Results of decisions at locations of interest are combined to give a graded risk for the structure as a whole as well as identifying specific locations of significant structural change. An operator is warned when a threshold risk is reached allowing response actions to be initiated, for example: safety procedures and/or repair procedures.