Investor-State Dispute Settlement and Constitutional LawInvestment treaties allow foreign investors to claim damages against states before tribunals of investor-state dispute settlement (ISDS). More frequently, such dispute settlement procedures tend to replace proceedings before national courts. This has given rise to the heated debate surrounding the ongoing negotiation about the free trade agreements between the European Union and the United States of America. This article identifies and discusses the constitutional law implications of such tribunals. The composition of the tribunals of private persons, the lack of a legal ground for public policy reasons to override investors' rights, the dynamic development of the adjudication based on vague legal terms and the lack of publicity and transparency in the proceedings – all this raises questions from the perspective of democratic principle and rule of law. Based on democratic principle doctrine, this article classifies rulings of tribunals as acts of public authority and highlights the lack of material and personal legitimacy and examines whether a state monopoly of adjudication can be derived from the separation of powers principle. It discusses the publicity and control of ISDS tribunals as an obligation enshrined in the democratic principles and highlights the missing legal reviewability of ISDS rulings compared to tribunals established under German administrative law. Finally, the article explores possible compensatory instruments addressing the identified deficits based on an application of investments treaties in line with constitutional law principles.