This chapter explores how Pius XI’s social encyclical Quadragesimo anno – “On the Reconstruction of the Social Order”– not only reiterated Leo XIII’s condemnation of socialism and his critique of aspects of capitalism, but also outlined a program for Catholics to follow in order to address the social and economic upheavals of the time in a lasting and far-reaching manner. It also analyzes the two primary contributions of Quadragesimo anno to Catholic social doctrine. The first concerns Pius XI’s articulation of two principles of Catholic social teaching: subsidiarity and social justice. The second contribution is the encyclical’s articulation of a very substantial prescription for fundamental social change. While the developments at the level of principle introduced by Quadragesimo anno have proved lasting and become a set fixture of Catholic social doctrine, we observe how the particular proposals associated by Pius XI with these principles– most notably, the development of vocational groups and the establishment of a type of corporatist social order– had, by the time of Saint John XXIII, been considerably relativized by the magisterium.