This report has reviewed the scope of non-invasive flow metering by considering those methods currently offering this possibility, and then considering other methods which might be developed to provide the same facility. For each technology the pattern has been to review the existing literature, identify key questions and consider development possibilities. This approach has included: ultrasonic flow metering, acoustic flow metering, the use of existing pipework, other approaches such as neural networks, tracers, cross-correlation. It also includes other flowmeter types in current use such as: vortex, electromagnetic, Coriolis and thermal flowmeters. In addition it has considered probes and tracers and some other less promising methods. The report has recommended directions for further work to optimise the R&D process, and has prioritised a work programme. In this, priority has been proposed for the two technologies currently offering non-invasive flow metering: ultrasonic and acoustic. It has also considered how to develop the use of pipe bends for this purpose. It is considered that, without access to pressure tappings, the use of pipe bends is a very much more uncertain approach. In conclusion the common concerns of internal pipe cross-section, essential for virtually all methods of non-invasive flow measurement, high temperature, and whether access is possible, were highlighted and a three pronged development proposed with ultrasonic, acoustic and pipe bends. However, it also recognises that there are possibilities with other methods, particularly neural network methods which should not be overlooked, and may need careful consideration. The report includes a list of references, and appendices relating to the initial specification of this report, fuller summaries of some references and a note on the correction factor for clamp-on ultrasonic flowmeters.