Abstract: This Article posits that international law on investment protection develops as a common law through adjudication of investor-state disputes. It reviews the three prevalent theories on the development of international law on investment protection. These three theories are (a) that investor-state decisions reflect a new customary international law, (b) that investor-state decisions are a potentially corrupt tool of corporate usurpation of international law, and (c) that investor-state disputes form part of a self-contained legal regime. The Article explains that each theory fails because it superimposes policy preferences not present in investor-state decisions. In rejecting these theories, this Article argues that investor-state disputes trace a case-by-case common law process rather than conform to any rigid theory. Accordingly, this Article provides a cogent theory of persuasive precedent in investor-state arbitration premised upon a common law understanding of persuasive authority in the U.S. courts. The case-by-case common law approach clarifies the current problem of substantively inconsistent decisions arising out of investor-state disputes. Normatively, the decision-making divergence between investor-state tribunals is preferable to artificial uniformity that the three currently prevalent theories impose upon investor-state decision-making tribunals and outcomes.