The problem of the lack of a definition of ‘investment’ in the ICSID Convention has been partially solved through the development of the so-called ‘Salini criteria’. However, the ICSID case-law and the most recent BITs show that the ‘Salini criteria’ may no longer be sufficient to cover the wide range of economic activities that States and investors deem to be worth of treaty protection. In the absence of a definition of “investment" in the Convention, the cases decided by ICSID arbitrators have followed one of two trends: objective, which determines the investment question according to objective criteria (investor assets, project duration, risk borne by the investor), and subjective, based on the parties’ agreement to qualify an economic operation as an investment. This paper provides a certain assessment of the Model BITs, ICSID case law, and scholarly contributions in respect of the legal meaning as well as interpretation of the notion of "investment" within the range of international investment law and provisions of the ICSID convention.