The FloodScale project is a contribution of the French hydrological community to the HyMeX program. Its general objective is to improve the understanding and simulation of the hydrological processes leading to flash floods. To reach these goals, a combined multi-scale observation and modelling effort is proposed, relying on nested-catchments instrumentation covering the following scales: (i) the hillslope scale, where process understanding on runoff generation and concentration can be tackled; (ii) the small to medium catchment scale (1-100 km2) where the impact of network structure, rainfall, landscape, initial soil moisture variability can be quantified; (iii) the larger scale (100-1000 km2) where transfer phenomena in rivers and flooding become important. The potential of innovative observations at various scales, complementing traditional measurements will also be assessed. FloodScale focuses on the Gard and Ardèche catchments of the Cévennes-Vivarais Hydrometeorological Observatory (OHM-CV). It builds on existing observation networks to propose (i) an harmonization of existing instrumentations; (ii) an enhancement of the measurements to better understand active hydrological processes during and between flash floods; (iii) innovative approaches for the documentation of ephemeral flash floods events; (iv) the evaluation of new data analysis and modelling approaches at various scales. The presentation will give an overview of the experimental set up and associated modelling approaches which will be deployed in FloodScale during the HyMeX EOP 2012-2015. http://www.mf-congress.com/hymex2012/abstracts/abstracts_print.php