Farming systems modelling provides reliable quantitative economic data on the different sources of income (from farming and off-farm activities), net return to investment, return to labour, and margins per activity. The Olympe software was developed by INRA/ESR, IAMM and CIRAD to model and simulate how farming systems function. The model is linked with a socio-economic analysis and takes all contextual situations - into account. It enables identification of farmers' strategies and trajectories. Simulations permit prospective analysis based on the volatility of prices and/or the impact of climatic events. This software was first developed in close cooperation with a number of researchers from research institutions involved in tropical agriculture. It was then used by the researchers for different case studies in North and West Africa, Southeast Asia and Latin America, with a wide variety of results and methodological applications. The variety of the results highlights the richness that can be expected from reliable and representative studies of farming systems at different scales: simple comparison between two cropping systems, characterisation of farming systems followed by monitoring or counselling, the study of entire irrigated schemes at the regional scale, prospective analysis and the elaboration of potential scenarios, and finally role plays. These very different applications can be used by different stakeholders, i.e. farmers, project or community leaders, extension institutions, and researchers, as well as policy makers.