The WTO Secretariat describes sustainable development as a central WTO principle. Relevant international law treaties have declared sustainable development's mutual supportiveness with trade liberalization, and also emphasized the need to balance its 'pillars': economic development, often equated with trade liberalization, with environmental conservation and social welfare. While 'mutual supportiveness' suggests that sustainable development's environmental and social goals are a side effect of trade liberalization, 'balancing' involves weighing these different goals, and prompts the difficult question of which are most important, and who is empowered to decide. This paper traces these two broad theoretical conceptions through WTO legal texts, negotiations and dispute settlement, arguing that they have important pragmatic implications. In particular, to create mutual supportiveness WTO Director-General, Pascal Lamy, has stated the need for adequate domestic policies, suggesting that the WTO should support these. Yet, if they have negative trade impacts, pure 'sustainable development' policies may be difficult to balance against the WTO obligation to liberalize trade. * Email: emily.lydgate@kcl.ac.uk. I am grateful to my Ph.D. supervisor, Federico Ortino, as well as the editor of the World Trade Review, Alan Winters, and two anonymous reviewers, whose comments have improved the quality of the article. Any remaining errors are mine alone.1 Available on the WTO website at www.wto.org-english-tratop_e-envir_e-sust_dev_e.