From The Oxford Handbook of International Legal Theory (Florian Hoffmann and Anne Orford, eds, Oxford UP, forthcoming 2014). --- This paper, part of a larger work on international law theory, sketches some early lines of inquiry towards a theoretical understanding of international environmental law. As the body of international law regulating human interaction with the natural world, one might expect this branch of law to be a cornerstone of the international system. Yet in practice, international environmental law's reach is strikingly circumscribed. Little of the governance of natural resources, for example, is 'environmental'. Subsisting at the periphery, environmental law focuses on conserving particular (rare, exotic) species and 'ecosystems', and curbing certain kinds of pollution. Its principles are vague, peppering the margins of rulings within other judicial fora: it is quintessential soft law. In this paper, we suggest that international environmental law's dilemmas are due to two competing heritages. On one hand, this law enshrines the peculiar pantheism of the European romantic period, positing the 'natural world' as sacred, inviolable, redemptive. On the other, its main antecedents are found in colonial era practices, which provided the data for the earliest environmental science and a laboratory for prototypical attempts at conservation and sustainable development. Caught between irreconcilable demands, international environmental law struggles today to avoid utopian irrelevance or nugatory paralysis.