Digital signal processing can help cut losses from leaking water mains. The authors explain how one new development, sponsored by Thames Water, is helping it in the battle to locate leaks more quickly and reliably. From the outset their aim was to develop a `virtual' instrument approach based on a standard personal computer (PC). In this way, as the knowledge base increases and further improvements are made, software upgrades can be implemented on the same hardware platform. The use of a standard commercially available sound card fits in with this approach. In practice the software development proceeded in two main stages: an initial programme of algorithm development, followed by real-time algorithm implementation and graphical user interface (GUI).