In 2018, the ICJ rendered a judgment in Bolivia v. Chile that effectively denied the status of the doctrine of legitimate expectation as a customary international law. The ICJ’s judgment came as a surprise to many in the international arbitration community because a whole host of international tribunals established under various investment treaties have found that this doctrine, as well as the broader principle of “fair and equitable treatment,” has effectively attained the status as the “minimum standard of treatment” under customary international law. Given the lack of elaborated reasoning, however, the ICJ’s ruling fails to resolve the recurring debate over the legal status of the doctrine. This paper addresses this issue squarely, by first examining the historical development and the nature of the doctrine, and then the conflicting lines of jurisprudence that arose out of a number of investment arbitrations. Thereafter, this paper attempts to provide an answer to the longstanding question as to whether the doctrine of legitimate expectation has now attained status as customary international law. Finally, based on a systematic analysis of various investment treaties and numerous arbitral awards from the perspective of public international law, this paper tackles the old conundrum by providing a pragmatic guidance on ascertaining applicable legal tests for the “minimum standard of treatment” under contemporary customary international law.