The joinder or intervention of third persons/addition- al parties in an arbitration proceeding has been dealt with procedur- ally under the rules of leading arbitration institutions in recent years. Although these rules vary from one institution to another, they are similar in terms of their basic perspective. This is because the increase in the number of multi-party and multi-contract disputes makes joinder and intervention of third persons/additional parties a necessity in the resolution of these disputes, and arbitral institutions have introduced provisions in this direction in their rules, making it possible for third parties to be joined or intervene in the proceed- ings. This study analyses the joinder/intervention provisions under the rules of the leading arbitral institutions including the Arbitration Rules of Istanbul Chamber of Commerce Arbitration and Mediation Center and compares these provisions to determine the conditions for the joinder or intervention of third persons/additional parties in an arbitration proceeding.