The concept of unjust enrichment is usually referred to as a general principle of law. This is supported by comparative legal analysis. Remedies against unlawful shifts of assets exist in nearly all contemporary domestic legal systems. Also international arbitral tribunals accept unjust enrichment as a general principle of law in their case law; as did the Iran-US Claims Tribunal in its jurisprudence. Thus, there is undeniable support for the claim that unjust enrichment constitutes a general principle of law. Still, the exact contours of the concept of unjust enrichment remain disputed. Unjust enrichment is not a unitary concept. This presentation argues accordingly that one has to look into each specific fact situation to see whether a claim based on unjust enrichment is admissible. This will be shown with particular focus on contracts/treaties tainted with illegality.