This thesis provides an analysis of the complex relationship between law and legitimacy in the WTO. It focuses on the notional dichotomy between ‘Member-driven’ (input-based) and ‘results-oriented’ (output-based) narratives of the WTO’s legitimacy, and how such narratives are both framed by, and reflected in, WTO law. It demonstrates how these narratives are used to legitimate the exercise of legal power in ways that exceed the reach of their internal normative claims; how they are used to displace responsibility for decision-making in the WTO; and the consequences of choosing to emphasize particular forms of legitimacy for our understandings of the WTO’s place in the world. In the process, the thesis also seeks to destabilize these legitimacy narratives by highlighting their partial, contingent and often mutually contradictory natures. The thesis proceeds in three parts. The first part (Chapter Two) clarifies what is meant by the terms ‘power’ and ‘legitimacy’ as used in the thesis and stresses their significance for WTO law. The second part (Chapters Three and Four) addresses two key input-oriented narratives of legitimacy associated with WTO law — those of consent and democracy. It argues that although consent has been central to understanding the legitimacy of WTO law as it is, and democracy is increasingly advanced in relation to WTO law as it should be, both narratives suffer from serious normative and descriptive limitations. The third part delves further into the concept of output legitimacy and its limits (Chapter Five), before exploring its application in relation to the legal-institutional dynamics of WTO negotiation rounds (Chapter Six) and the treatment of economic evidence in WTO dispute settlement (Chapter Seven). This part ultimately concludes that a more critical engagement with the concept of output legitimacy could open up productive avenues for rethinking the law and practice of the WTO.