Justice, both as a universal concept and as a real state of affairs in the world, is central to Christian theology and Christian ethics. We shall employ the framework of the Catholic tradition primarily in this examination. This emphasis is not to suggest that other traditions—Christian or otherwise—are irrelevant to the analysis of justice. Rather, our aim is simply to recognize the contributions of the oldest philosophical legacy on the subject of justice that, although obscured by contemporary justice theories, still stand securely by their logical scaffolding. In Catholic teaching specifically, the classic definition of justice advanced by Thomas Aquinas is to render to another his due. This definition serves as the foundation in Christian social thought upon which the notions of rights (as entitlements), of right conduct, and of rightness of a situation can be understood. In other words, a person’s due is what he is entitled to. Those actions that are directed toward securing for a person his due constitute right conduct. And the end-state in which the person has been served his due through the right conduct of others that made this possible, is a right situation and, thereby, a just state of affairs. This monograph attempts to unravel the distinct applications of social justice, economic justice, and distributive justice in modern Christian social thought. The purpose is to set up a framework for justice that properly distinguishes each type, and also clarifies the relationships between instances of these types.