The constant evolution of the ways in which foreign capital is invested in the marketplace has urged the need for defining the boundaries of the notion of investment, as Article 25 of the International Centre for Settlement of Investment Disputes (ICSID) Convention is silent on this point. Against this background, the question of whether the category of portfolio investment, which generally consists of intangible capital flows, can be classified as protected investment under the ICSID Convention has gained momentum due to some high-profile ICSID cases. This article examines to what extent portfolio investment can be included in the notion of investment under Article 25(1) of the ICSID Convention and, consequently, the substantive protections granted thereto can be enforced within the ICSID framework. It is argued that the increasing sophistication of financial instruments makes it difficult to find a one-size-fits-all solution, thereby requiring a case-by-case assessment of the relevant financial instrument vis-à-vis the typical characteristics of an investment identified by the case law. In this analysis, the requirement of the ‘contribution to the economic development of the host state’, which has often been overlooked, is subject to reconsideration.