Home to more than half the U.S. population by 1920, cities played an important role in the development of American religion throughout the 20th century. At the same time, the beliefs and practices of religious communities also shaped the contours of America’s urban landscape. Much as in the preceding three centuries, the economic development of America’s cities and the social diversity of urban populations animated this interplay. But the explosive, unregulated expansion that defined urban growth after the Civil War was met with an equally dramatic disinvestment from urban spaces throughout the second half of the 20th century. The domestic and European migrations that previously fueled urban growth also changed throughout the century, shifting from Europe and the rural Midwest to the deep South, Africa, Asia, and Latin America after World War II. These newcomers not only brought new faiths to America’s cities but also contributed to the innovation of several new, distinctly urban religious movements. Urban development and diversity on one level promoted toleration and cooperation as religious leaders forged numerous ecumenical and, eventually, interfaith bonds to combat urban problems. But it also led to tension and conflict as religious communities busied themselves with carving out spaces of their own through tight-knit urban enclaves or new suburban locales. Contemporary American cities are some of the most religiously diverse communities in the world. Historians continue to uncover how religious communities not only have lived in but also have shaped the modern city.