The concept of “odious debt” regroups a particular set of equitable considerations that have often been raised to adjust or sever debt obligations in the context of political transitions, based on the purported odiousness of the previous regime and the notion that the debt it incurred did not benefit, or was used to repress, the people. This paper begins with an exploration of the grounds of the “odious debt” concept in basic international law structures and principles. The international law obligation to repay debt has never been accepted as absolute, and has been frequently limited or qualified by a range of equitable considerations, some of which may be regrouped under the concept of “odiousness.” This is consistent with the accepted view that equity constitutes part of the content of “the general principles of law of civilized nations”, one of the fundamental sources of international law stipulated in the Statute of the International Court of Justice. At the same time, most debt contracts between States and private creditors are governed by the domestic private law specified in the contract. The legal systems of these jurisdictions may well have concepts such as “clean hands” or the notion that contracts related to illegal purposes are invalid. These concepts overlap with elements of the notion of “odiousness” as a basis for invalidating debt obligations. Investor/state arbitration tribunals, for example, have been comfortable taking into account such considerations in determining whether repudiation of contractual obligations to an investor by the host State is consistent with international law. This suggests that such concepts may indeed form part of the content of equity as “a general principle of law of civilized nations”, especially if widely shared among different legal systems.