This article discusses partnerships between nato and other international institutions, with a particular focus on questions of international responsibility raised by such partnerships. It concludes that ‘partnership’ is a notion that is used by nato in a rather general sense, rather than as referring to a clearly delineated category with specific legal connotations. In practice, nato has partnerships with States, international organizations, ngos and private actors. Such partnerships are relatively little formalized, including where questions of responsibility are concerned. As a consequence of the lack of specific arrangements between nato and partners, and the lack of practice—or in any event practice that has been made public—the ars and ario have a potentially important role to play. Upon closer scrutiny, however, the application of these principles raises many questions. One reason for this is that they are relatively underdeveloped in respect of situations where multiple actors are involved.