This chapter outlines past and current definitional issues at the nexus of energy poverty, energy vulnerability, energy justice and energy transitions. It traces the historical development of scientific understandings centring on these topics, while exploring their interactions and interdependencies. The chapter starts from the multiple definitional controversies surrounding fuel poverty and energy poverty, to then discuss the different ways in which notions of energy vulnerability and energy justice have enriched traditional understandings. The latter has been achieved, in part, thanks to a fuller appreciation of the services and production chains via which energy circulates across territories.