This chapter explores the various methods by which investment tribunals ascertain the applicable law. The most common approach is to follow the legal submissions presented by (one of) the parties. At times, external expertise—in the form of legal experts or amici curiae—is also required, particularly when the tribunal is unfamiliar with the applicable law. Moreover, a tribunal may take ex officio initiatives to ascertain the law, such as conducting independent research, offering autonomous interpretations, or even introducing new legal sources.