The Mediterranean countries are experiencing important challenges related to the water cycle including water shortages and floods, extreme winds and ice/snow storms that impact critically the socioeconomic vitality in the area (causing damage to property ; threatening lives ; affecting the energy and transportation sectors, etc.). There are gaps in our understanding of the Mediterranean water cycle and its dynamics, which include the variability of the Mediterranean Sea water budget and its feedback on the variability of the continental precipitation through air/sea interactions, the impact of precipitation variability on aquifer recharge, river discharge, soil water content and vegetation characteristics specific of the Mediterranean basin and the mechanisms that control the location and intensity of heavy precipitating systems which often produce floods. The HyMeX (Hydrological cycle in the Mediterranean Experiment) programme is a 10-year concerted experimental effort at the international level aiming at advancing the scientific knowledge of the water cycle variability in all compartments (land, sea and atmosphere) and at various time and spatial scales. It also aims at improving the processes-based models needed for forecasting hydro-meteorological extremes and the models of the regional climate system for predicting regional climate variability and evolution. It finally aims at assessing the social and economic vulnerability to hydrometeorological natural hazards in the Mediterranean and the adaptation capacity of the territories and populations therein to provide support to policy makers to cope with water related problems under the influence of climate change, by linking scientific outcomes with related policy requirements.