The Mediterranean region is frequently affected by heavy precipitation events (HPE) associated with flash-floods (FF), landslides and mudslides each year that cost several thousands millions of euros in damage and causing too often casualties. Within the framework of the 10-year international HyMeX program dedicated to the hydrological cycle and related processes in the Mediterranean, a major field campaign has been dedicated to heavy precipitation and flash-floods from September to November 2012. The 2-month field campaign took place over the Northwestern Mediterranean Sea and its surrounding coastal regions in France, Italy and Spain. The observation strategy aimed at documenting four key components leading to heavy precipitation and flash-flooding in that region: (i) the marine atmospheric flows that transport moist and conditionally unstable air towards the coasts; (ii) the Mediterranean Sea as a moisture and energy source; (iii) the dynamics and microphysics of the convective systems; (iv) the hydrological processes during flash-floods. The presentation will briefly provide the rationale for developing this field campaign and an overview of the design and execution of the field campaign (Ducrocq et al, 2013). Highlights on specific Intense Observations Periods (IOPs) will illustrate some first results using aircraft and ground-based observations and associated modeling studies in terms of (i) atmospheric process understanding, (ii) HPE and FF predictability studies and (iii) NWP validation and improvement. HyMeX is endorsed by WWRP-THORPEX and WWRP JSC.