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 thro...