International courts and tribunals are increasingly faced with a number of technical, extralegal issues in disputes before them, often entire disputes hinging on technical questions. Consequently, the international dispute settlement system is witnessing a gradual alteration from a procedural viewpoint. As a tool to assist the adjudicator in decision-making, the ‘expert’ has entered the international judicial process, playing varying roles in various disputes. This chapter looks at disputes where both parties are states. The first section compares the criteria and method of selection of experts by the adjudicator in various fora; this is followed by a study of procedures involving an ex curia expert, and a comparative analysis of procedures involving party-appointed experts. The chapter concludes with a brief overview of the possibilities for further lessons from a broader comparative review.