This chapter is an invited contribution to the first English-language comparative study of subsidiarity, M. Evans and A. Zimmerman (eds.), Subsidiarity in Comparative Perspective (forthcoming Springer, 2013). The concept of subsidiarity does work in many and varied legal contexts today, but the concept originated in Catholic social doctrine. The Catholic understanding of subsidiarity (or subsidiary function) is the subject of this chapter. Subsidiarity is often described as a norm calling for the devolution of power or for performing social functions at the lowest possible level. In Catholic social doctrine, it is neither. Subsidiarity is the fixed and immovable ontological principle according to which the common good is to be achieved through a plurality of social forms. Subsidiarity is derivative of social justice, a recognition that societies other than the state constitute unities of order, possessing genuine authority, which which are to be respected and, when necessary, aided. Subsidiarity is not a policy preference for checking power with power. This chapter traces the emergence of the principle of subsidiarity to the neo-Scholastic revival that contributed to the Church's defense against the French Revolution's onslaught aimed at eliminating societies other than the state. The concept of subsidiarity has implications for the present, changing socio-political landscape in the United States as the Church faces a state that is poised to compel the Church to violate the moral law.