The Fifth International Conference on Hydroinformatics, Hydroinformatics 2002, was held from July 1st - 5th 2002 at City Hall, Cardiff, UK, and was opened by her Royal Highness, The Princess Royal. The conference was organised in association with the International Association of Hydraulic Engineering and Research (IAHR), the International Water Association (IWA) and the International Association of Hydrological Sciences (IAHS). Hydroinformatics is a relatively new discipline concerned with the application of computer and networking technology for the planning, management and protection of the water environment. Its aim is to allow environmental managers and engineers to make rapid and robust decisions as they address the increasing challenges of ensuring a sustainable water environment and adequate water resources for future generations. Hydroinformatics 2002 addressed these objectives with over 250 high quality presentations, by authors from over 50 countries, organised under the following themes: Decision support and management systems; Integration of technologies; GIS; Tools, Environments and Languages; Numerical engines; Data acquisition and management; Data mining; Neural networks in hydroinformatics; Internet, Intranets and Extranets; Inverse modelling and data assimilation; Uncertainty and risk; Ecology and water quality modelling; Experiences with modelling systems; Evolutionary algorithms; Policy and Strategy support systems. Hydroinformatics 2002 provided an excellent opportunity for engineers, environmental managers, scientists to meet and exchange ideas in this rapidly developing field. These proceedings will provide an invaluable reference to researchers, practitioners and all those interested in the field of Hydroinformatics.