The aim of this book is to identify an emerging public-law concern: the review of the administrative actions of a host state in investment arbitration. This book examines the extent to which the principles of domestic administrative law can be used as a legal reference for investment arbitrators to address and resolve the legal issues presented in regulatory matters that are part of investor-state treaty arbitrations. In arriving at an answer to this particular concern, two factors are considered: (i) the use of administrative law principles as a part of the unitised nature of the law that governs the ‘state of law’ of any democratic society; and (ii) the current crisis of legitimacy that the investor-state treaty arbitration system is facing.