Even today, the focus for leak detection programs in water delivery systems has been primarily on distribution networks and service connections. Trunk main leaks tend to be addressed only when there is a pipe rupture or when there is visual evidence of the leak above ground. The introduction of tethered and autonomous in-line leak detection technologies has the potential to resolve the uncertainty about the rate of loss from trunk mains. Experience to date has demonstrated that these are valuable tools for identifying, locating and quantifying leakage. Results indicate that trunk main leakage is indeed a concern for many agencies, and that most leaks are not evident from visual inspection. This paper presents an overview of the operation of the in-line leak detection systems and summarizes experience to date from many kilometers of trunk mains inspection and also covers latest technological improvements to in-line autonomous and free swimming leak detection tools.