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. We briefly elaborate on the conceptual foundations of the studies of intermediaries in transitions, and how intermediaries have been connected to different transition theories. This shows the divergence – and sometimes a lack – of conceptual foundations in this research. In terms of transitions theories, many studies connect to the multi-level perspective and strategic niche management, while intermediaries in technological innovation systems and transition management have been much less explored. We aim to bring more clarity to the topic of intermediaries in transitions by providing a definition of transition intermediaries and a typology of five intermediary types that is sensitive to the emergence, neutrality and goals of intermediary actors as well as their context and level of action. Some intermediaries are specifically set up to facilitate transitions, while others grow into the role during the process of socio-technical change. Based on the study, as an important consideration for future innovation governance, we argue that systemic and niche intermediaries are the most crucial forms of intermediary actors in transitions, but they need to be complemented by a full ecology of intermediaries, including regime-based transition intermediaries, process intermediaries and user intermediaries.