The common good (bonum commune) has, since antiquity, referred to the aim of social and political association, and was particularly prominent in medieval Christian political theology. Since St. John XXIII’s 1961 encyclical letter, Mater et magistra, ecclesiastical statements about social teaching have employed a formulation of the common good, usually in the version that appeared in the Second Vatican Council’s 1965 Pastoral Constitution for the Church in the Modern World, Gaudium et spes, as “the sum of those conditions of social life that allow social groups and their individual members relatively thorough and ready access to their own fulfillment.” This chapter discusses the origins and development of this formulation as well as the ways that it has been used in subsequent Catholic Social Teaching. While it has sometimes been interpreted as an “instrumental” account of the common good, the sources and uses of the notion suggest that it is the particularly modern political component of a fuller notion of the common good continuous with the tradition. In particular, the recent formulation is concerned to limit the power of the modern state and protect the dignity of the human person in the challenging conditions of political modernity.