What is the status and role in public action of the knowledge possessed by 'simple' citizens, users and professionals? That is the question broached in both this article and the entire special issue for which it serves as the introduction. To this end, we explore the abundant scientific literature pertaining to the topic and try to situate our own position within the broader setting. After discussing the gradual questioning of the social representations that have made scientific knowledge the

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... eal and standard by which we measure all knowledge, we argue that many authors with an essentialist approach to knowledge have stressed the differences between scientific knowledge and non-scientific knowledge, often leaving us at an impasse. We argue therefore that it is preferable to advance an approach in which knowledge is as at once relational and in a constant process of hybridization. Having opted for and justified this position, we then focus on the -hybrid -knowledge possessed by citizens, users and professionals, by first probing the reasons for the growing involvement of these actors in the production of knowledge and policies. We then ponder the nature and foundations of the complaints and criticisms frequently levelled at participatory mechanisms as to the actual role played in these areas by the knowledge held by 'local' actors. In the end, we identify proposals defended by certain authors to make the interactions of actors from different social worlds more symmetrical.