How transparent is the international investment law regime, and how transparent should it be? Most studies approach these questions from one of two competing premises. One camp maintains that the existing regime is opaque and should be made completely transparent; the other finds the regime sufficiently transparent and worries that any further transparency reforms would undermine the regime’s essential functioning. This paper explores the tenability of these two positions by plumbing the precise contours of transparency as an overarching norm within international investment law. After defining transparency in a manner befitting the decentralized nature of the regime, the paper identifies international investment law’s key transparent, semi-transparent, and non-transparent features. It underscores that these categories do not necessarily map onto prevailing normative judgments concerning what might constitute good, bad, and murky transparency practices. The paper then moves beyond previous analyses by suggesting five strategic considerations that should factor into future assessments of whether and how particular aspects of the regime should be rendered more transparent. It concludes with a tentative assessment of the penetration, recent evolution, and likely trajectory of transparency principles within the contemporary international investment law regime.