Existing theories of WTO law cannot adequately explain the form or content of the GATT exceptions, in particular Article XX(a) Public Morals. Nor, in consequence, can they satisfactorily answer the interpretive questions they raise. This article explains Article XX in terms of self-determination as a political and moral value, and the choices it mandates peoples make for themselves. Drawing on debates in contemporary political philosophy, it distinguishes three categories of argument for self-determination: intrinsic, expressive and instrumental, each having implications for the scope of the choices a self-determining community must make for itself. This account of self-determination in trade regulation is used to reconstruct Article XX, both explaining the individual provisions, and suggesting how these might be developed and interpreted. It concludes by examining Article XX(a) in detail, highlighting the interpretive questions public morals pose, and how understanding Article XX in terms of self-determination suggests these should be answered.