This chapter examines the notion of legal knowledge, which lies at the linguistic core of the iura novit curia principle. It explores whether investment tribunals are subject to mandatory requirements or, at least, held to certain expectations of legal knowledge. To this end, the chapter provides an overview of the various normative sources relevant in investment arbitration and assesses the extent to which arbitrators are expected to be familiar with them. The analysis covers sources of public international law, procedural knowledge and skills, as well as domestic law. Finally, the chapter proposes a model clause outlining knowledge requirements that could be incorporated in future investment treaties.