Abstract : The Naval Facilities Engineering Service Center (NFESC), Port Hueneme, California, and its industrial partners, Vista Engineering Technologies, L.L.C., and Vista Research, Inc., have demonstrated and validated (an innovative, mass-based leak detection system for aboveground storage tanks (AST). The Low-Range Differential Pressure (LRDP) system is a computer-controlled system that can reliably detect small leaks in ASTs, which range from 50,000 gal to ASTs with diameters of over 260 ft containing over 10,000,000 gal of petroleum fuel. While no specific national regulatory requirements presently exist for ASTs, stringent state requirements are forcing Department of Defense (DoD) facilities to take their tanks out of service to install double bottoms and perform interstitial monitoring. With a validated, high-performance, in-tank leak-detection system for ASTs, like the LRDP, an alternative strategy is now available that is cost-effective and does not necessitate taking the tank out of service. The results of the evaluation showed that the LRDP has the performance to meet the monthly monitoring and annual precision (tightness) test regulatory compliance requirements set for bulk underground storage tanks (USTs) using a test that takes less than 24 h to conduct. This project was performed under the Environmental Security Technology Certification Program (ESTCP). The objective of the ESTCP is to demonstrate and validate innovative environmental technologies needed to address the environmental objectives of the DoD, are cost-effective and will be ready for the development of commercial products and services at the completion of the demonstration/validation (DEM/VAL). All the objectives of the project have been met, and the LRDP is ready for and currently in commercial use. Both on-line, permanently installed monitoring and testing systems and tightness testing services using the LRDP can be obtained commercially through Vista Research.