The principle of subsidiarity is a multi-layered and flexible principle that can be utilised to empower, inform, enhance and reform scholarship in a range of significant areas, however, it has been somewhat overlooked in recent scholarship. In order to highlight the continued relevance and potential applications of the principle, this, the first of two papers, will provide a detailed analysis of the meaning and application of the principle of subsidiarity in Catholic social teaching. In doing so, the interplay of the principle of subsidiarity and other key principles of catholic social teaching such as dignity of the person, solidarity, and the common good will be highlighted. The second part of this paper discusses the political applications of the principle, including its ability to inform scholarship on the allocation of governmental powers (including federalism), democracy, and individual participation in government. This leads to a discussion in the second paper, of the Catholic aspects of subsidiarity in the governance of the European Union