Spain has a tradition of water management and allocation based on a dual model of water rights, where surface water is public, but groundwater has historically been considered a private resource. The Water Law (1985) created a new concessional system for the assignment of water rights to users of both resources, controlled by the State, but preserving the historical rights. Water allocation, in practice, is determined by the resources and demands that are estimated in the water planning documents. Therefore, water allocation is granted by River Basin Authorities and depends on water availability, social and environmental priorities, and the system operating rules. It is an institutional model of water allocation, under State control, but partially open to participation and negotiation with users. The main weakness of the Spanish water allocation model derives from the application, during the 20th century, of an excessively generous policy of water rights allocation and the lack of control of water uses. This policy has led to the overallocation of water rights and groundwater overexploitation in some basins, generating structural deficits, dramatic environmental impacts and significant social and territorial tensions. This context, which hinders the implementation of environmental flows, has required the design of different programmes and plans to control water uses, and the introduction of new legal instruments to stimulate water rights temporal exchanges.