The meaning of the term “investment” is a key jurisdictional issue in investor-State arbitration and lies at the heart of the international law of investment claims. In the absence of a protected “investment”, ICSID arbitration simply cannot take place. As a matter of fact, article 25 of the ICSID Convention is silent at defining this basic term and the qualification for transnational arbitral protection depends on the definition provided by the applicable investment treaty. Notwithstanding, the lack of consistent interpretation has generated a broad debate. Legal doctrine and arbitral practice have developed a set of “criteria” and “tests” in order to determine whether a particular investor–State dispute arises out of an authentic “protected investment”. This has led to a tripartite conceptual approach: an objective, a subjective and a dual meaning of the notion of “investment” in international investment law. This article traces the trajectory of the definitional approaches as applied by ICSID and non-ICSID tribunals and seeks to shed light on the future prospects of the fundamental concept in the face of the evolving investment treaty regime.