Introduction -- Investor-State Arbitration and the Argentine Cases: an Overview -- Proportionality as a General Principal of Law -- Proportionality and the Concept of Systemic Integration -- Proportionality and Expropriation -- Proportionality and the FET Standard -- Proportionality and NPM Clauses: Article XI of the Argentina-US BIT -- Proportionality and the Customary International Law Defence of Necessity -- Summary and Concluding Remarks.