Meteorological and hydrological aspects and hydraulic problems related to extreme floods which occur in arid regions of Spain are reviewed. Small coastal basins with steep slopes and little vegetation cover are characteristics of these regions in which "flash floods" develop. They are produced by large convective rainfall, resulting in very intense soil erosion and deposition. The flow in alluvian cones is basically two-dimensional, unsteady, and often supercritical. Using mathematical models which incorporate these features are typically prohibited because of the cost associated with topographical precision requirements. Since a large number of dams has been constructed and are operational in Spain, management of reservoirs considering the potential of flood events has become an important problem in Spain.