In this paper, I argue that the inherent powers of ICSID Tribunals are broad both in prescriptive terms and have been descriptively treated as such in arbitral decisions and State practice. The argument will be presented in three steps. First, a brief overview of the nature of inherent powers in international adjudication will be given (Part II). It will be suggested that inherent powers may be derived from a multiplicity of partly overlapping sources. While inherent powers may be subject to limitations (just as any other non-peremptory rules of international law), treaty makers can introduce limitations only by express treaty or arbitral rules. Secondly, the general proposition will be applied to ICSID Tribunals, explaining the source and limitations of their inherent powers (Part III). Thirdly, particular aspects of inherent powers of ICSID Tribunals will be discussed in greater detail, considering in turn the powers to grant binding provisional measures, to deal with abuse of process, to accept submissions of non-parties, to deal with evidence and to order remedies, and the powers operating specifically at the annulment stage (Part IV). It will be suggested that Tribunals and States have treated the inherent powers of ICSID adjudicators of investment disputes as broad and rightly so: the general methodology and its application in particular instances go with the grain of established and perfectly unremarkable international practice on these matters.