This doctoral thesis addresses the role of the written and oral pleadings submitted by States in international disputes in the process of treaty interpretation, arguing that the contents of State party pleadings concerning treaty interpretation can and often should be taken into consideration by adjudicators/interpreters under certain provisions in the articles of treaty interpretation in the Vienna Convention on the Law of Treaties (VCLT) – namely, VCLT Articles 31(1), 31(3)(a)-(c), 31(4) and 32 – as well as certain additional conduits, i.e. evolutionary interpretation, authentic interpretations, estoppel and admissions against interest. The dissertation also aims to establish that State party pleadings are a form of State practice that is not a suspect class of interpretative material. The doctoral work further argues that tribunals should – and sometimes must – give due accord and deference to the content of such agreed interpretations, and that the failure to do so may lead to continued backlash against the international dispute settlement regime (especially investor-State) by some States.