Within the HyMEX project, the focus is put on two different hydrological aspects at the regional scale i) the understanding and modelling of flash-floods ii) the long term simulation of the water balance and of its dry extremes: the draughts. For each case study, dominant hydrological processes and the spatial and temporal scales at which they must be represented are different. The simulation of flash-flood is driven by an accurate representation of runoff generation, which depends on antecedent soil moisture, slope, surface features and rainfall intensity. Evapotranspiration is one of the major components of a long term water balance. Its simulation must take into account the heterogeneity of land use, the temporal variability of climate conditions and the modifications induced by human activity, especially for the simulation of draughts. The presentation will provide a synthesis on spatial and temporal characteristic scales of hydrological processes, of their observation and of their representation within hydrological models. Existing hydrological modelling efforts within the Mediterranean region will also be overviewed. The specificity of the Mediterranean region, will be pointed out, focusing on the: - Role of geomorphologic, climatic and anthropogenic factors such as the basin size size (large vs small basins), the topographic position (mountainous vs plain basins), the hydrogeologic and aquifer system (e.g. specific processes in karstic regions), the urbanization, islands, the role of dams and reservoirs, the lakes, and human activities such as agriculture, irrigation, hydroelectricity, industrial activities. - Links between model and experimentation: data required as input or validation variables such as hydro-meteorological data, geographical data, remote sensing, tracers.. - Parameterisation, calibration and validation strategies. - Coupling hydrologic models to water resources management and socio-economic models to simulate the impacts of land use changes