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. We 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. We integrate existing conceptual models on transition dynamics and phases and a typology of transition intermediaries to examine how intermediaries advance transitions in different phases. We illustrate our 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 predevelopment, to the aggregation of knowledge, pooling resources, network building and stronger institutional support and capacity building in acceleration. Highlights • An integrated model of intermediation in different transitions phases is created. • Empirical examples from automobility, heat pump and low-energy homes transitions are given. • Without intermediary influence, pre-development is unlikely to lead to acceleration. • Powerful intermediaries employ functions of visioning, networking, institutional support, and capacity building. • Policy action should map if relevant intermediaries exist and are functional enough.