Bolstered by a wave of immigration in the early 20th century, a polyglot American Catholic Church struggled to forge a secure subculture in a society that regarded Catholics with suspicion. In the decades that separated the Roaring Twenties from Vatican II, Catholic Action inspired laypeople to participate in the work of the Church's hierarchy. In endeavors that ranged from religious education and liturgical renewal to labor activism and immigrant outreach, this movement permitted the Church to maintain its distinctiveness while simultaneously engaging with the wider American culture. Contributors use archival research to describe and interpret an array of lay movements across the United States during the middle decades of the 20th century.