This thesis develops a reliance-based theory for protection of legitimate expectations in investment treaty arbitration based on the moral philosophy of promise. It critically assesses the current approaches on the doctrine of legitimate expectations which focuses on sources of law. It proposes an alternative framework based on first principles in moral philosophy to evaluate the core of legitimate expectations and discusses three possible conceptions of legitimate expectations. First, the voluntarist theory of legitimate expectations attributes liability to the State’s intention to undertake an obligation. Second, the assurance (bare-expectation) theory focuses on the voluntary act of creating expectations. Third, the reliance theory emphasises the act of detrimental reliance by the investor on the state’s promise as the basis for liability. The thesis defends the reliance conception as the unifying conception and examines its three normative implications: purpose of the principle, conditions for liability and remedy. First the theory underpins the fundamental purpose of the principle of legitimate expectations with compensation of harm suffered by the investor rather than the enforcement of sovereign promise. Second, the theory provides a doctrinal structure for considering conditions for liability. Third, the theory reinforces the argument for reliance damages. The thesis articulates a normative framework for adjudication of claims for breach of the fair and equitable treatment standard.