The essay explores how ideas about social justice and economic performance shaped the debates over federal taxation in the United States since the origins of the republic. The debates were most intense during major national emergencies (the American Revolution, the Civil War, World War I, the Great Depression, and World War II), and each debate produced a new tax regime-a tax system with its own characteristic tax base, rate structure, administration apparatus, and social purpose. The criterion of "ability to pay" and a concern for economic efficiency powerfully shaped the formation of every tax regime, but "ability to pay" became the more influential of the two considerations during the national crises of the twentieth century.