Detailed Knowledge on local farming systems and farmers' strategies in various situations such as pioneer zones, rehabilitation areas or traditional tree-crop belts) may contribute to build better and more adapted alternatives, solutions and proposals to help farmers to make the right decision at the right time concerning their future investment. CIRAD has developed (with INRA and IAMM) a software called "Olympe" that enable the modelling of farming systems. There is also a module that permit the analysis at the level of farms groups. Positive or negative externalities can be integrated. The use of "Olympe", a farming system modelling tool is aimed to improve farmers' understanding of their own situation, of their socio-economic context as well as to provide orientations for agricultural and development policies for institutions or donors. Modelling farming systems results can be used (beside other uses) as a prospective tool to build scenarios about potential farms trajectories. It might be used to the definition of agricultural policies, recommendations domains, and measuring impacts in order to addres policies to farmers' reality.