Book excerpt: Despite the Lisbon Treaty reforming the EU Treaty provisions on external relations, it was argued at the time of the Treaty’s entry into force that ‘mixity was here to stay’. While this has indeed proven to be the case, the Court of Justice’s jurisprudence has nonetheless redrawn the contours within which mixity can thrive and for the first time has confirmed the existence of ‘facultative mixity’. In light of these significant post-Lisbon developments the volume aims to clarify the law and policy of facultative mixed agreements in the EU’s treaty practice and this not only from the perspective of EU (constitutional) law itself but also from the perspective of the EU Member States’ legal systems, that of the EU’s third country treaty partners and that of public international law itself.