This collection of essays constitutes the fruit of a scholarly conference held in Rome in April 2023, which was organized by the Pontifical University of Saint Thomas Aquinas, the Pontifical University of the Holy Cross, and the Angelicum Thomistic Institute. The essays offer a scholarly reflection on various historical and doctrinal aspects of Aquinas's concept of ius, and on the extent to which this concept helps to illuminate his account of the nature of law, that is, of the juridical phenomenon. Each essay addresses, from the viewpoint of its specific topic, the implications of the Angelic Doctor's description of ius as the object of justice, as presented in the Secunda secundae of his Summa theologiae (starting from question 57, article 1). Following Aquinas's own insights, the focus on the virtue of justice is thus deemed crucial for understanding the essence or the ontological fabric of law. Justice is simultaneously analyzed as always remaining inherently connected to its moral and divine-salvific aspects. The volume is divided in three parts. The first part of the collection contains essays that are dedicated to the analysis of Aquinas's concept of ius in a historical perspective. The second group of essays in this collection examine some further doctrinal and contextual issues pertinent to the essence of ius. Each attempt to elucidate the ontology of law should be supported by the aptitude of the resulting accounts of law's nature to provide workable platforms for legal practices. This line of reasoning represents the rationale for the third part of this collection, which focuses on the matters of applying Aquinas's concept of ius in systemic and practical legal settings of State law and canon law.