In recent years there has been a noticeable increase in references by investor–State arbitral tribunals to general principles of law as a source of law, however these references have rarely been accompanied by detailed explanation. This book aims to provide the actors involved in investor–State arbitration with a set of comprehensive guidelines to better understand the nature, meaning, and function of general principles of law in the field of international investment law. Applying these principles to practice, the book assesses seventeen concepts and notions in the field of investment arbitration to provide counsel and arbitrators with clear guidance on what should (and should not) be considered a general principle of law.