The past decade has seen outside states, coalitions or regional organisations lead military operations at the (real or alleged) ‘invitation’ of one of the parties embroiled in military strife – notably, in Mali, Syria, Yemen, and The Gambia. This chapter addresses key legal questions, starting with the validity of a government’s ‘consent’. Others include the legal nature and the effects of such consent, and how a government might lose its capacity to consent to military action in its own territory. The chapter includes analysis of the legal doctrines that might limit the lawfulness of the intervention despite such consent, including the invalidity of consent, the ‘negative equality’ doctrine or complete abstention, unlawful purposes, and other grounds of illegality, such as the inviting state’s complicity in international crimes. In the cases examined in the volume, the intervening states typically invoked multiple legal grounds, of which invitation is only one, and the UN Security Council has been intensely involved: the chapter appraises the legal significance of these novel features. It concludes with a summary of the trialogue method and an overview of the subsequent chapters.