This chapter considers the criteria for collective self-defence that are shared with individual self-defence. It is uncontentious to say that the same criteria that apply to individual self-defence – armed attack, necessity, proportionality, the reporting requirement, and the ‘until clause’ – also apply to collective self-defence. Indeed, this is an inevitable consequence of the way the concepts appear in Article 51 of the UN Charter. The nature and application of these criteria in the context of individual self-defence have been examined at great length in the existing literature. This chapter therefore does not provide in-depth analysis of all of their aspects. That said, it does provide a brief overview of these requirements to ensure that this book presents a comprehensive picture of the operation of collective self-defence today. The chapter’s main focus, though, is to examine how the operation of these criteria works specifically in the context of collective self-defence actions, which is something that has been largely overlooked in scholarship.