The chapter documents the complex relationship between, on the one hand, equity and international law and, on the other, principles and rules, and it discusses equity as a source of international law. The thrust of its argument is that equity is a source of international law, which means that international adjudicators have the power to apply it. While conceding that the felicitous conjunction of law and equity presents a formidable challenge, the chapter suggests that equity can only be conceived as forming part of the law. After discussing equity embedded in conventional law, the chapter turns to customary international law, general principles of law, and general principles of international law. It puts forward the argument that equity is a general principle of international law of a customary law nature, having mutated from a general principle of law through its repeated use at the international level.