This book, written from a practice-oriented perspective, examines whether and how questioning from the Bench actually achieves its objective, i.e. to assist the Court. Particular circumstances such as incidental proceedings and the time factor have an impact on the act of questioning, while the provisional relevance of issues raised—perceived as such by both the Court and the parties—may later turn out to be effective and rather important. Judicial actors have raised both conceptual and contextual questions, as well as procedural and substantive, and even hypothetical ones. The subject-matter of questions depends on the Court’s champ opératoire, while the parties’ litigation strategy and their co-operational duties provide the framework to respond to the Court and to comment on the other party’s replies. A survey of this practice—by both the judicial actors and the parties—well-established now after almost eight decades, constitutes the groundwork for the qualitative analysis of the manner in which questions, replies and comments have found their way into the judicial reasoning of the Court and of individual judges. In the overwhelming majority of judgments and individual opinions this judicial engagement has been co-decisive. This is the first, in-depth, and comprehensive monograph providing a functional analysis of this managerial tool at the Court’s disposal, making it essential reading for both scholars and practitioners, studying and involved with the Court’s jurisprudence. Karel Wellens is Emeritus Professor of International Law at the Radboud University Nijmegen in The Netherlands.