A description is given of the application of the systems approach to the planning of a large catchment basin in central Italy. The aim of the planning was to ensure the optimal satisfaction of water requirements for drinking water, industrial and irrigation purposes, as well as to provide flood control and protect the quality of the water in the river and its tributaries from pollutants.A hydrological study was carried out, using existing discharge data recorded at some gauging stations, and pluviometric data, in order to assess the succession of the average monthly discharges in a large number of sections and for a contemporaneous period of about 50 years. A second hydrological study was made to identify the frequency distribution of the floods and the mechanism by which they were formed and propagated.All sites which previous studies had indicated as potentially suitable for flood protection works were examined, together with others that were selected directly. A feasibility study was made for each of these, the results being used to estimate the law of costs for the works in function of the available storage capacity. With regard to water consumption the requirements of drinking water were calculated, since this was considered a constraint of the system. The requirements for irrigation and industrial use were also calculated, together with the benefits accruing.An analysis was made of the damage caused by floods in the recent past together with an extrapolation of the estimate of this damage in the future.In addition, an assessment has been made of the effects on the dispersal of the flood water and the costs of local protection works such as embankments and diversion channels.As regards pollution, the potential quantity of pollutants produced upstream of each control section has been estimated, as well as the cost of their total or partial elimination. The standards for the maximum permissible pollution in the sections were also fixed.Using the above inputs an optimization process in mixed linear programming was carried out, using the hydrological flows of a year considered significant. This made it possible to identify the optimal degree of satisfaction of water requirements, the optimal degree of flood control for the various places in danger, the degree of purification needed to satisfy the standards, bearing in mind the dilution resulting from the regulation of the discharges. A selection has been made of the works required for the achievement of the above aims at minimum cost.The basin scheme corresponding to the indications provided by the optimization process has been submitted to a simulation which utilized all the available hydrological data, i. e. for a period of about 50 years.In a successive phase the irrigation of a vast district (that the optimization process had not accepted) was considered as a constraint and the related social costs were evaluated.