Recently, increasing attention has been paid to intermediaries, actors connecting multiple other actors, in transition processes. Research has highlighted that intermediary actors (e.g. innovation funders, energy agencies, NGOs, membership organisations, or internet discussion forums) operate in many levels to advance transitions. The authors argue that intermediation, and the need for it, varies during the course of transition. Yet, little explicit insight exists on intermediation in different transition phases. It is integrated existing conceptual models on transition dynamics and phases and a typology of transition intermediaries to examine how intermediaries advance transitions in different phases. Tha authors illustrate their conceptual insights through examples from car clubs, heat pumps and low-energy housing. We conclude that intermediation is paramount from predevelopment to stabilisation of a transition. Intermediary functions change from supporting experimentation and articulation of needs in pre-development, to the aggregation of knowledge, pooling resources, network building and stronger institutional support and capacity building in acceleration