Intermediary actors have been proposed as key catalysts that speed up change towards more sustainable socio-technical systems. Research on this topic has gradually gained traction since 2009, but has been complicated by the inconsistency regarding what intermediaries are in the context of such transitions and which activities they focus on, or should focus on. This paper aims to bring more clarity to the topic of intermediaries in transitions, providing a typology that, beyond functions of intermediaries, is sensitive to different levels and phases of transitions, and the origin and emergence of intermediary actors. Based on a systematic review of academic scholarly articles, the paper identifies five different categories of intermediary actors that play a role in transitions based on their level of operation and origin, being insider/outsider, the level of agency, and the degree of neutrality. Some intermediaries are specifically set up to facilitate transitions, while others grow into the role of an intermediary during the process of sociotechnical change. On the basis of the study we argue that systemic and niche intermediaries are most crucial forms of intermediary actors in transitions, which should be also considered in planning future innovation governance frameworks. The paper further elaborates how intermediation occurs in pre-development, take-off, acceleration and embedding, and destabilisation phases. We note the lack of literature about intermediary activities in different phases of transition, in particular with respect to the acceleration and embedding phase. We, thus, suggest what kind of transition intermediaries are needed to better support the acceleration and embedding of innovations contributing to long-term sustainable development.