The turn of agrarian sciences and agricultural extension from reductionist and transfer of technology, respectively, towards systemic approaches has transformed agricultural/rural development thinking in the last decades. Nevertheless, the emergence of Agricultural Innovation Systems (AIS) has to confront a number of gaps among which the expert – lay knowledge gap is of major importance. This paper aims at exploring such a gap as well as obstacles to participatory development from a critical realist point of view. Critical realism (CR) with its realist, differentiated and stratified ontology aims at interpreting the world in order to ultimately bring about transformation. CR allows for new insights on the nature of knowledge as well as on development research and practice. It thus provides useful guidelines concerning the emerging ‘intermediation’ functions within AIS.