General principles of law play an important role in investment arbitraion and can be applied by a tribunal when no treaty provision or rule of customary international law exists regarding a particular issue. They can be used in traditional means, such as the interpretation of vague treaty terms, or for wider reaching issues emerging from the international legal order. Following a significant increase in references to the general principles of law by Investor-State tribunals questions have been raised around the meaning and function of these principles. Written by an expert in the field this book offers clear and comprehensive guidelins 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, this book assesses 17 concepts and notions in the field of investment arbitration, providing counsel and arbitrators with clear guidance on what should, and should not, be considered a general principle of law.