This is a practical handbook for scientists, analysts, and students working on the definition and implementation of policy changes that contribute to real economic and social agriculture-based development, with a special focus on commodity systems. The objective is to promote efficient and sustainable agrifood systems that broadly benefit society. The method presented here relies on the participation of actors in directing the evolution of agrifood systems, including both identifying problems as well as elaborating policies and implementing decisions through concrete actions. There are two linked components: consensus-building between actors to define actions and to implement changes, and participative research based on commodity-chain analysis methods. The first part of the book shows how to promote a dialogue and the consensus-building process among the various actors of a commodity system. It is based on the output of a thorough analysis of the systems situation, conducted in a participatory way. This analytical approach is detailed in the second part of the book with reference tables, examples and hints for data collection, analysis and interpretation.