Casanova applies his customary historical-sociological analysis to the question of "What is a public religion?" in this book chapter. He quickly states that both "public" and "religion" are sociologically problematic terms that need significant differentiation. After an attempt to nuance both of these broad terms, Casanova goes on to examine the processes of the modernization and interaction of "public religions" -- notably Christianity and Islam -- in the American context. He regrets that, while the US is turning itself via immigration into the world's pre-eminent global society, Islam is still persistently viewed by Americans as an "other" religion. He closes with a hopeful vision that, no matter how fraught dialogue appears now, Christian-Muslim understanding may bear fruit as Islam shows Americans how it has resisted racialization and the American environment pushes Islam to evolve both in the US and abroad, as was the case with Judaism and Catholicism in America in the past. This chapter was published in Religion Returns to the Public Square: Faith and Policy in America (2003, eds. Hugh Heclo and Wilfred M. McClay).