This book offers a comprehensive study of the principle of good faith, a frequently invoked but rarely analysed aspect of investment arbitration. It considers the application of good faith by arbitral tribunals and parties in international investment disputes, encompassing both procedural and substantive aspects of good faith. Expertly negotiating a complex principle, the book diligently follows the arbitral process from jurisdiction through merits and to cost decisions, identifying the various applications of good faith in investment disputes. It examines the role of good faith in defining nationality and investor as well as pre-dispute admissibility requirements. It also delves into the ways the principle guides parties’ arguments and informs tribunals’ decisions regarding evidence, substantive protections, and costs. It further addresses the role of good faith in the behaviour of arbitrators and other actors involved in the investment arbitration process.