This book provides a conceptual and legal analysis of the core of investment protection guarantees that emerge from international treaties signed since 1959 for the promotion and protection of foreign investment. It focuses on both the origin and evolution of investment treaty standards. Beginning with origins, the work considers the broader context at the time when the first modern investment treaty was concluded. It goes on to examine the many decisions of ad hoc arbitral tribunals that have since been called upon to apply these treaties in order to resolve the several hundred investor-State disputes. It also looks at some of the recent investment treaties that have attempted to clarify and/or reform the content and scope of investment protection guarantees. Federico Ortino posits that the key investment protection provisions in investment treaties, and thus much of the controversy associated with such treaties, revolve around three concepts: legal stability, investment's value, and reasonableness. He argues that, from the very beginning, the protections afforded to foreign investments by modern investment treaties have been exceptionally broad, and as such restrictive of host States' ability to regulate. And whilst a growing number of investment treaty tribunals, as well as new investment treaties, have to some extent reined in such broad protections, the evolution of key investment protection standards has been marred by inconsistency and uncertainty.