Arbitration tribunals do from time to time find themselves confronted with situations under which they are obliged to equate two or more competing legal principles. A fundamental issue thus manifesting itself is how such operations should be undertaken. A range of methods, otherwise known as “standards of review”, have both been proposed and is utilized in practice. One particular alternative is known as “proportionality”. This thesis aims to assess to what extent proportionality is, and should be, preferred as the applicable standard of review within the field of law known as international investment law. The latter currently exhibits a somewhat confused state of affairs regarding this issue, a situation that is becoming more and more intolerable as the economical importance of international investment law continues to accelerate.