The development of organic farming is nowadays seen as a possible solution to meet the different objectives of sustainable development. In the Camargue, South of France, environmental issues are strongly linked with agriculture, and stakeholders are in need of integrated approaches to analyse the plausible impacts of alternative farming systems at multiple scales, particularly at regional scale. Evaluating alternative farming systems at multiple scales requires the quantification of different indicators meaningful for different actors in support of the negotiation and decision making process. With this challenge in mind, we developed a framework for Prospective, Integrated, Multiscale and Participative Assessment of Agricultural Systems (PIMPAAS), combining modelling tools and stakeholder meetings. In this paper we present the proposed framework based on the possible complementarities of two modelling approaches using scenario analysis (Bioeconomic models and multi-agents systems) for a multiscale, integrated and participatory evaluation. We also present the progresses made in the identification of scales of analysis in relation to actors' objectives and aspirations, in the description of current and alternatives farming systems and in the identification of useful indicators at different scales. The list of the main stakeholders is presented and the objectives and consultation procedures are highlighted with examples. First results concerning the use of crop models are also presented. We conclude on the generic aspects of the approach for a prospective, integrated, multiscale and participatory evaluation of the extension of alternative agro-ecosystem such as organic farming.