Several authors have described the usefulness of participatory system dynamics approaches in environmental decision-making processes, particularly in supporting problem scoping and policy analysis. This paper explores how these approaches may be expanded to provide a coherent, deliberative platform, which structures Integrated Sustainability Assessments (ISA) of policy proposals. The proposed ISA framework forms a five-stage feedback and learning process including scoping, visioning, model building, simulation/assessment and monitoring. We discuss these elements considering a set of best practice principles for participation in environmental assessment and decision making. Subsequently, we lay out a roadmap for implementing and testing the framework. We conclude that participatory modelling has a strong potential for supporting ISA processes. It allows stakeholders to build alternative policies, to reflect on their long-term dynamics, and to gain insights on the interrelationships underlying persistent sustainability problems.