Mechanisms for individuals to bring claims under international law have become increasingly common in recent decades, particularly in human rights and investment law. Nonetheless, when the International Law Commission codified the law of State responsibility, it largely ignored the bringing of international claims by individuals, and the relationship between such claims and those brought on the interstate level. Overlapping Individual and Interstate Claims in International Law is the first dedicated monograph examining this relationship - one that is of mounting importance on both a practical and theoretical level. This work provides a comprehensive survey of the potential for overlapping individual and interstate claims to arise. It underlines issues of fairness, consistency, and interference with autonomy that can result when multiple claimants vie to have their claims determined before different forums. The author analyses in detail how treaty provisions and various rules and principles of international law can be expected to regulate such overlapping claims, considering, among others, the local remedies rule, the rule precluding double recovery, res judicata, waiver, and certain circumstances precluding wrongfulness. The book clarifies the nature of international claims, including in the theoretically muddled field of diplomatic protection, and highlights undertheorized foundations of topical debates concerning the use of countermeasures and self-defence outside of the interstate arena. It concludes with a human rights-oriented proposal for resolving the complex policy issues to which these overlapping claims give rise.