To the date, Budget Plan definition in Italian public companies is approached with insufficient deepness. We believe this is common to other countries too.Indeed, most public organizations develop the Budget Plan basing themselves only upon the available data and not upon knowledge acquired in years of experience. Balance sheet data are therefore obtained simply adding to the previous information, an amount estabilished, for example, on the expected inflation rate.This approach although “trivial” supports the budget responsible, because during the budget presentation, very few elements can be effectively criticized. It is commonly accepted that also public companies find themselves in turbulent environments. This is due to both the increased number of endogenous variables, and to the complexity of exogenous parameters. Therefore the Budget Plan definition becomes always more critical, and consequential difficulty of its evaluation assumes relevant importance.The paper describes an experiment carried out by an Italian Public company which is adopting a dynamic economic- financial model for both, Budget Plan definition and for its evaluation. The model is based upon System Dynamics approach and evaluates a series of scenarios providing support to the budget definition responsibles in taking strategic policy decisions, and better “explaining” the effects of decisions undertaken.