Part of the allure of Jessup’s description of transnational law undoubtedly lies in its promise of capturing something beyond the most visible aspects of public and private international law – in its invocation to uncover ‘unseen law’, including law that is important in practice but neglected in scholarship. Transnational law, whether viewed substantively or as a methodological approach, expands law’s vision field. Yet the task of making visible the actualities of law and practice comprising transnational law involves slippery methodological questions that legal scholars seem particularly skilled at sidestepping. This short chapter examines the intellectual holding pen created by Jessup for other rules and sources of law, his ‘larger storehouse of rules’. It interrogates the meaning of the practice-enriched perspective that transnational law claims to deliver, arguing that those who invoke practice in fact mean many different things. Finally, the chapter considers the twin research challenges of selective vision and partial knowledge, identifying four ‘black boxes’ that might inspire new directions in transnational environmental law research.