This article seeks to clarify how international courts and tribunals should decide whether to exercise jurisdiction over incidental issues. It considers such issues incidental, which would fall outside the subject-matter jurisdiction of an international court or tribunal if submitted separately, but which courts rule upon to resolve disputes falling within their jurisdiction. International courts and tribunals have employed diverse approaches to decide whether to exercise jurisdiction over incidental issues. This contribution will assess their decisions to distil what criteria are best suited to ensure the effectiveness of the underlying treaty while taking into account the fundamental importance of state consent to judicial dispute settlement. It concludes that the necessity to exercise jurisdiction over the incidental issue and the nature of the issue should be the guiding criteria for international courts and tribunals, while the character of the jurisdictional basis may serve as supplementary criterion.