The most important aquifers of the Llobregat river basin are located in its lower part, namely the Low Llobregat Aquifer System (LLAS). Intensive groundwater development since the late nineteenth century and mostly after the 1950s has dramatically changed its hydrological functioning. Groundwater quality has suffered from salinity increase due to the potash mining area in the centre of the basin and seawater intrusion, as well as agriculture, waste disposal, sand and gravel pit backfilling and leakages. A stricter management is reversing and improving the situation and thus preserving the role of the LLAS as a crucial factor for the water supply to the Metropolitan Area of Barcelona. Besides model-based groundwater quantity management and stricter rules to protect water quality, additional actions include artificial recharge and installing a barrier to halt seawater intrusion by injecting highly treated reclaimed waste water. A groundwater user community was created and plays an important role in moving towards a sustainable use.