The process of jurisdictional regulation in investment treaty arbitration is governed by the principle of consensualism, which in theory confers the power to define the jurisdiction of arbitral tribunals on disputing parties exclusively. The practice of arbitral tribunals, however, has given the impression that tribunals are willing to establish jurisdiction on alternative and weaker bases than party consent. This book addresses the question to what extent party consent indeed defines the jurisdiction of arbitral tribunals. It argues that the consensualism of investment treaty arbitration is eroded by the rise of arbitral law-making in the matters of jurisdiction. The book tracks the development of specific arbitrator-made jurisdictional rules, which direct the interpretation and application of party-provided jurisdictional rules but also impose additional jurisdictional limits. The development of such rules decreases the share of disputing parties in the process of jurisdictional regulation. The book addresses the need for the recognition of the regulatory function of arbitral tribunals and its integration into the jurisdictional framework of investment treaty arbitration. To that end, the book sketches a two-layered model of jurisdictional regulation in investment treaty arbitration, consisted of both party-defined and arbitrator-made rules. The recognition of arbitral law-making as a pertinent part of the jurisdictional structure of investment treaty arbitration would provide more legal certainty in the creation and application of arbitrator-made rules, and possibly rebuild consensualism, by allowing for a clear coordination between the two layers of jurisdictional regulation.