This chapter examines the iura novit curia principle as applied by international courts and tribunals operating under public international law, as well as in international commercial arbitration. It argues that while international courts and tribunals generally embrace the application of iura novit curia, the principle is met with scepticism in the context of international commercial arbitration. This divergence stems from the distinct structural features that characterise each procedural framework. These differences are particularly significant for the ensuing analysis of iura novit curia in investment arbitration, which is shaped by elements of both paradigms.