This chapter seeks to delineate the notion of collective self-defence in international law. While the core concept can be stated relatively easily, there has been persistent controversy regarding the nature of collective self-defence. It is possible to identify no fewer than five different ‘conceptions’ of collective self-defence that have been advanced in scholarship. These conceptions are all explored in detail. The chapter also examines the question of whether collective self-defence is indeed an ‘inherent right’, as Article 51 of the United Nations Charter proclaims. The status of collective self-defence as a right (and, moreover, as a right that is inherent) has been contested. As such, its status requires theorisation based on the analysis of the views of states. Finally, the chapter considers the modality of collective self-defence: in other words, it asks what ‘qualifies’ as an act of collective self-defence. In examining this question, there is particular focus on whether the provision of weapons and logistical support in support of an attacked state amounts to the exercise of collective self-defence.