This chapter examines the relationship between collective self-defence and another legal basis for the use of force, which in scholarship is referred to as ‘military assistance on request’ or ‘intervention by invitation’. Analysing the relationship between collective self-defence and military assistance on request is crucial because these concepts are, in some respects, strikingly similar. Indeed, it has been argued that they overlap, and states often blur them in their argumentation. The chapter explores the extent to which the concepts can be differentiated at the ‘doctrinal’ or ‘conceptual’ level. It then turns to the various legal requirements (actual or, in some cases, arguable) for collective self-defence and military assistance on request, with the aim of highlighting similarities or differences, as relevant, when it comes to the operation of these two concepts.