It will be shown, how cost-efficient flow metering in water mains and water networks are realized with non-invasive ultrasonic flow measurements. A brief introduction of the measurement technology will be given. There are several factors, which makes clamp-on ultrasonic flow measurements especially challenging. Mains can be buried, submerged, exposed to fluctuating ambient conditions. It will be explained, based on several successful international projects, how FLEXIM solves the unique technical challenges encountered when monitoring the flow of main trunks.