Compressed natural gas (CNG) is being increasingly used as an automotive fuel. This gas is stored in fuel cylinders at pressures up to 3600 psig to maximize the vehicle range. These cylinders must be inspected periodically; hydrostatically and/or visually, which requires removal of the cylinders from the vehicle. An alternative technique termed SLAM (for source location acoustic monitoring) has been used here to inspect cylinders in the US Postal Service fleet. The advantage of the SLAM technique is that the cylinders were inspected without removing them from the vehicles. The inspection was performed during refueling and resulted in minimal vehicle down time.The SLAM technique involves placing sensors on the cylinder surface and measuring the acoustic activity emanating from defects. The SLAM technique captures the total acoustic emission (AE) waveform, unlike other parameter based AE techniques that capture simple measures such as AE counts or hits. The advantage of this approach is that the maximum amount of information can be extracted from AE activity such as the location of the acoustic events. The technique can detect most of the critical forms of damage to CNG cylinders, such as fatigue cracking of metal cylinders or impact damage in composites - that may not normally be detected with visual or hydrostatic tests. The ability to locate the source is particularly important in identifying damage - since most damage is localized. The theory and background of the SLAM technique is described. The results obtained from the inspection of 50 Postal Service vehicles are presented together with a cost benefit analysis between the SLAM method and the conventional hydrostatic test.