ABSTRACTThe article begins by exploring the crisis of contemporary multilateralism in terms of three core elements: institutions, norms and negotiation. It then argues that in relation to these elements, the EU and the US occupy ambiguous positions and roles, and that these ambiguities, exacerbated by internal tensions and crises as well as external forces, pervade their changing roles in the multilateral system. The third part of the paper assesses the role of multilateralism in the development of transatlantic relations, drawing attention to the differentiation between ‘special relationships’, modes of transatlantic governance and broader multilateral mechanisms, and to the ways in which this reflects the underlying ambiguities in the roles of the EU and the US. The paper concludes that the consequent tension between differentiation and disintegration will be a core element of transatlantic relations for the foreseeable future, and that EU-US relations are thus weakening in a foundational aspect of world order.