The Catholic Church has been the largest single religious denomination in the United States since the mid-nineteenth century, consistently claiming between 20 and 25 percent of the nation’s population over the past 170 years. Arriving in the Americas with the earliest Spanish and French missionaries, following pathways of settlement and expansion to all corners of the United States, and bolstered by successive waves of immigration, both in the past and today, Catholicism has been a constitutive element of American history from the very beginning. It has shaped the development of the nation in myriad ways, from formal political engagement and social activism to subtle cultural influence and anonymous works of charity. It is also one of the nation’s most internally diverse religious denominations, drawing together members from all races and ethnicities, socioeconomic classes, ideological perspectives, and political persuasions. American Catholics espouse a wide range of beliefs, customs, and practices that reflects the church’s cultural richness, but that has also given rise to numerous clashes and outright conflict. They continue to maintain a strong sense of collective unity, yet also exhibit much of the same polarization found within wider society. It is a church that today is at once growing and contracting, vibrant yet visibly scarred by scandal. For all these reasons, American Catholicism captivates and confounds, and it deserves scholarly attention.