The COST 717 action is implemented by three working groups, each investigating different aspects of the use of radar observations in hydrological and NWP models. Working Group 1 (WG-1), the subject of this paper, looks at the use of radar observations in hydrological models. It has divided the work into 9 subject topics each being undertaken by teams of 2, 3 or 4 members of the WG. Initially these focus on (i) identifying the current state of radar use in Europe and compiling a database of operational systems from which data may be obtained or research undertaken, and (ii) on identifying the sensitivity of hydrological model outputs to the type of high resolution, spatially distributed, precipitation inputs which can be provided by radar. This is the first step in identifying the need for and benefits from such inputs. Account will be taken of the differences between urban and rural catchment requirements. Once these have been done, WG-1 will investigate the use of combined radar/ hydrological modelling systems and these together with NWP systems.