This chapter explores the development of modern CatholicCatholic thought on taxation, tracing its evolution from Pope Leo XIII’s 1892 encyclical, Rerum Novarum, to contemporary U.S. Catholic theological and episcopal reflections. Although Vatican pronouncements on taxation have remained limited, Catholic social teaching situates tax policy within a framework emphasizing the principles of subsidiarity, solidarity, and the preferential option for the poor. Using the United States as a case study, the chapter examines how these principles have been interpreted and applied by theologians, bishops, and Catholic activists such as Dorothy Day. It analyzes key moments in magisterial and theological discourse and argues that the preferential option for the poor mediates the enduring tension between subsidiarity and solidarity in tax discourse, and that modern Catholic social teaching increasingly recognizes tax policy as a moral issue—one that calls for just distribution, moral compliance, and practical concern for society’s most vulnerable. The chapter concludes with a call for more robust theological engagement with tax policy in light of its foundational role in advancing social justice in contemporary society.