Users Guide to Ecohydraulic Modelling and Experimentation has been compiled by the interdisciplinary team of expert ecologists, geomorphologists, sedimentologists, hydraulicists and engineers involved in HYDRALAB IV, the European Integrated Infrastructure Initiative on hydraulic experimentation which forms part of the European Community’s Seventh Framework Programme. It is designed to give an overview of our current knowledge of organism-environment interactions in marine and freshwater aquatic systems and to provide guidance to those wishing to use hydraulic experimental facilities to explore ecohydraulic processes. By highlighting the current state of our knowledge, this design manual will act as a guide to the use of living organisms in physical models and experiments and help scientists and engineers understand limitations on the use of surrogates. It incorporates chapters on the general decisions that need to be taken when designing an ecohydraulic experiment as well as specific chapters on the main aquatic and marine organisms likely to be of interest. Each of the chapters reviews current knowledge in a defined area of ecohydraulic experimental research. It excludes consideration of fish and mammals and does not deal with plankton, as it focuses on the sediment-water interface and the influences of biota in this complex area. Its primary purpose is to disseminate the extensive knowledge and experience of the team of ecohydraulic experimentalists involved in HYDRALAB IV as part of the PISCES research project as well as some of the important advances being made in this fast developing field of research.