This chapter addresses the parties to international arbitration agreements. It follows the standard distinction between signatories (those persons whose names appear on the arbitration agreement – or the contract that includes an arbitration clause) and nonsignatories (those persons whose names do not appear on the arbitration agreement but are nonetheless bound by or entitled to invoke it). Part I addresses the treatment of ‘parties’ in the New York Convention and national arbitration laws governing international commercial arbitration, and the ICSID Convention governing (many) investor-State arbitrations. Part II examines the theories under which affected others – that is, nonsignatories to an international arbitration agreement – might nonetheless be bound, likewise first in international commercial arbitration and then in investor-State arbitration. Finally, Part III discusses the available empirical evidence on the parties to international commercial arbitration and investor-State arbitration proceedings.