This paper considers under what circumstances, and for what reasons, an international tribunal may want to range beyond the primary text of a treaty to determine its ‘correct’ meaning; such extraneous legal material potentially including rules of customary international law, other treaties between the parties, general principles of law, and documents of a ‘soft law’ nature. The paper suggests a variety of ‘mechanisms’ by which a tribunal may undertake a broader interpretative approach, though all ultimately raise the same inevitable tension between accusations of judicial activism and counter-arguments of inflexibility and insularity. Nevertheless, many tribunals and individual judges continue to utilize such techniques, particularly noticeable in some recent environmental disputes. The paper will conclude with a note of caution; that though such interpretations are becoming an increasingly significant part of modern judicial decision-making—something that is generally to be welcomed—tribunals must concurrently take care to ensure that they remain within the accepted parameters of the adjudicative function.