Abstract: This chapter explores the role and limitations of judicial authority within the horizontal structure of international law. Unlike domestic courts within hierarchical systems, international courts (ICs) are not connected to centralised enforcement apparatuses and thus lack coercive power (‘potestas’). ICs rely instead on auctoritas – their ability to shape interpretations – to mobilise the pro-compliance forces in the international order. Authoritative communications reduce interpretative uncertainty, establish normative focal points, and act as a catalyst for sanctioning mechanisms. ICs may legitimise or mobilise sanctions across multiple layers of international law enforcement, including bilateral responses, third-party enforcement, and sanctions applied or authorised by international institutions. Through its expressive component, adjudicative authority not only impacts actors who willingly accept an adjudicator’s legitimacy; it also changes the strategic normative environment in which all actors operate.