This chapter considers the various functions of States in international commercial and investment arbitration, which are essentially twofold.First, States are legislators and the creators of the system, and they also assist its development through their conduct which informs the interpretation of relevant treaties, such as BITs.Secondly, States are also parties to such proceedings, usually as the respondent to a claim, but also possibly also as the claimant, as a counter-claimant, and as a non-party intervener.This chapter then considers issues faced by States when acting as the respondent, and discusses how States might best prepare for and handle such disputes, and it also identifies various procedural issues which are likely to arise in such arbitral proceedings.It concludes that States have a complex combination of roles in international arbitration, and that the challenges in responding to claims can be managed if States are willing to learn from the decades of experience which States now have of participating in international arbitration proceedings.

Specific procedures for third-parties joinder in arbitration rules are now included in most institutional rules, and stands out as a key recent development in arbitration procedure. This development highlights the system’s responsiveness and versatility in addressing users’ new needs to respond to the changes in business practice, which has become more complex and involved more parties and transactions.

The chapter provides an overview and assessment of the most frequently used rules, in both commercial and investment arbitration, including under the ICC, LCIA, CIETAC, SIAC, HKIAC and UNCITRAL, in relation to third-parties joinder. These rules have addressed in a variety of ways the central issue of consent of the parties to arbitration, which is a key foundation of international arbitration.

Moreover, the rules have also sought to continue to implement the principles of equality and neutrality, which are fundamental to arbitration and key reasons why parties select to go to arbitration in the first place. Thus, while addressing demands for flexibility and reconfiguration of a bi-lateral into a more multi-lateral system, the rules have also tackled and resolved appropriately the key problem of providing the parties with an equitable way to appoint arbitrators of their own choosing.