Petroleum tanks used to store crude oil, aviation fuel, gasoline, diesel and other products above and below ground. Leaking tanks may create significant environmental damage and jeopardize safety. Soil and water contamination can be tremendous and be undetected for years. Different methods have been developed and applied during the recent decades for non-destructive evaluation of metal and fiber-reinforced plastics (FRP) structures. Among these methods, acoustic emission technology is unique as it not only detects flaws but also is used for on-line, real-time monitoring of structural integrity without interruption of operation, cleaning, or product evacuation.