Research about the destabilisation, decline discontinuation and phase-out of existing systems in the scope of socio-technical transitions has developed significantly over recent years. A number of research orientations can be identified, addressing conceptual, empirical, policy and societal challenges in different ways. This chapter seeks to explore this variety, as a means to reflect on current and future research directions. It contributes to this recent development but also seeks to open up new perspectives. After clarification and disambiguation of related terms (destabilisation, decline, phase-out), the paper extensively reviews contributions to destabilisation research within transitions studies, structured around six salient themes as entry points for destabilisation research: lock-in, sources of change, incumbent actors, processes and mechanisms, governance, and vulnerability and politics. For each of these themes, the chapter discusses distinct research questions, conceptual elaboration and empirical strategies, as well as ongoing research puzzles. A discussion section formulates conceptual propositions for each theme. The chapter concludes by formulating unanswered questions as avenues for future research endeavours.