Investigations started 20 years ago in the Vallcebre research basins with the objective of better understanding the hydrological functioning of Mediterranean mountain basins. The Vallcebre basins are located in a Mediterranean mountain area of the Pyrenean ranges (northeastern Spain). Average annual precipitation is 862±206 mm and potential evapotranspiration is 823±26 mm. Climate is highly seasonal, leading to periods with water deficit in summer and, on occasion, in winter. Hydrological research in the basins is related to rainfall interception, evapotranspiration, soil moisture spatio-temporal dynamics, runoff response and runoff processes, suspended sediment dynamics and model application at both the plot and basin scales. Findings obtained over the last two decades have shown that, due to their intermediate position between drier and wetter climatic areas, the Vallcebre basins have a particular hydrological dynamic, which is specific to Mediterranean mountain areas.