This book explores the meaning of 'investment' within the context of International Centre for Settlement of Investment Disputes (ICSID) arbitration. It provides a comprehensive and detailed examination of the various legal issues arising in connection with the jurisdictional requirement of the existence of an investment. It explores, first of all, the fundamental question of whether the term 'investment' in Art. 25 ICSID Convention has - despite not being defined - some objective or independent meaning. Second, it addresses the substance of that meaning, showing that three main approaches (the prevailing Salini test, the permissibility test, and the commercial-transaction test) co-exist in arbitral practice. Third, it analyses the definitions of 'investment' found in investment treaties including the traditional definitional model, typical requirements, and recent developments in practice. Fourth, it provides an overview of definitions contained in domestic investment laws, highlighting commonalities with, and differences from, definitions found in investment treaties. Finally, it examines the investment status of several specific categories of assets and operations. The Concept of Investment in ICSID Arbitration offers not only a detailed analysis of the relevant case law, legislation, and scholarship, but also a critical assessment of existing practices and trends, as well as normative recommendations. It also explores issues that are neglected in the existing literature, such as the question of the nature of investment, recent trends in treaty drafting and arbitral case law, and definitions of 'investment' contained in investment laws. Despite its formal focus on ICSID arbitration, significant portions of the book are also relevant for other forms of investor-state arbitration.