This dissertation retrieves a nuanced, pre-conciliar account of the common good and develops it to offer descriptive insight and normative guidance for a post-conciliar and increasingly interconnected world. Working within the tradition of Catholic social thought, this project extends the concept of the common good to address the international sphere, the common goods that give rise to international society, and the laws of nations that guide and govern the pursuit of these goods.Current approaches to a globalized world present three interrelated difficulties: first, a lack of prescriptive guidance at the global level; second, an increasingly vague idea of the common good in the tradition of Catholic social thought; and third, an unfamiliarity with the pre-conciliar common good framework. This dissertation addresses these difficulties in reverse order.The first chapter argues for rediscovering the Leonine renewal as a mediating tradition that bridges the gap between Aquinas' thinking and our own, and between Leo XIII's social thinking and our own. The second chapter begins to retrieve the concept of the common good by identifying and sorting out various meanings of the term. The third chapter continues by showing how the primary sense of common good as a good, one in number and sharable by many, gives rise to an ordered array of various common goods, to which correspond various societies. Taken together, these first three chapters argue for and then carry out a retrieval of the framework developed within the Leonine renewal and a rejoining of this tradition with the recent revival of common good thinking in broadly Thomistic circles.With this reconstituted tradition in hand, chapter four develops and applies it to interpreting the description of the common good in Gaudium et spes, reading it in continuity with the earlier tradition but also as widening the scope of this tradition to grapple with an interconnected, globalized world. Chapter five embraces this widened scope to put forward an account of the common good of political society as the complete, temporal human good. Chapter six widens the scope still further to consider the distinction between the political common good and the international common good, focusing on law as the principal means within political society for achieving its common good and how this serves to differentiate it from international society and its laws. The common good framework both illuminates and is illuminated by broadly natural law-type arguments for the necessity and authority of international law and for its formation primarily through customary law. International environmental law illustrates this dynamic in a particularly helpful and urgent way.