This article addresses the tools by which international lawyers engage in interpretative ‘boundary crossings’ across distinct international regimes, such as those involving trade, investment, and human rights. It distinguishes the traditional tools of treaty interpretation, such as those licensed by the Vienna Convention on the Law of Treaties (VCLT), that encourage interactions between trade and international investment law (such as the VCLT’s Article 31(3)(c), from some more innovative interpretations now proposed by self-identified ‘public law’ scholars. Drawing on examples of boundary crossings pursued recently by investor-state arbitrators and the International Law Commission, it warns against interpretative boundary crossings that go against the object and purpose, remedies or organizational structures of the underlying regimes. It argues that such interpretative linkages, however well-meaning, may not be as ‘progressive’ as anticipated.