Current changes in the field of agriculture (e.g., food shortages, market globalization, and environmental concerns) are encouraging stakeholders to consider new and more sustainable production methods. To this end, tools are needed to assess the newly designed solutions, before taking them to the field (ex ante assessment), or after being implemented by farmers or by researchers on experimental sites (ex post assessment). MASC is a multi-criteria assessment tool designed to assess the performance of cropping systems in terms of their sustainability. It is based on a decision support system (DEXi) and aggregates 39 qualitative evaluation criteria arranged in a tree-like structure. In this paper, we present a case study involving farmers from Normandy (north-western France). MASC was used in a participatory approach with thesefarmers who were already working on reducing pesticide use. The overall performance of the different farmer’s cropping systems was evaluated. In comparison to similar models, MASC allowed the farmers to become involved in determining the weights allocated to different criteria, thereby enabling them to introduce their personal vision of sustainability into parameter settings. Moreover, rather than assessing at the farm or landscape scale, MASC made it possible to exchange and collectively work on the concept of sustainability at the field scale. In addition, the results provided by the model were collectively examined in order to identify the strengths and weaknesses as well as propose improvements on the assessed cropping systems.