The term “Adventist” embraces a cluster of fifteen or so Protestant religious communities in America for whom the central conviction in their belief system is the idea of the imminence of the Parousia, meaning the premillennial, personal, literal return of Christ to earth and the end of the evil world. The suffix, -ist, compounded with Advent, first used in the late 1830s to describe the sharp sense of imminence that motivated the movement, soon came to be used as the common term to identify those who held the conviction. By 1844 they were called Adventists. Almost all American Adventist churches are native to the United States and arose from the Millerite movement of 1831–1844 that marked the end of the Second Great Awakening (1790–1850). The churches it birthed were shaped by the cultural and religious impulses of the period. Most of the Adventist churches remain small, with an aggregate membership within the United States approaching 3 million. Worldwide Adventist memberships, however, exceed 40 million and represent a significant export of American religious beliefs and values. The two largest churches in the group, Jehovah’s Witnesses and the Seventh-day Adventist church, have both established a substantial presence in America, each reporting domestic memberships of approximately 1.25 million in 2018, placing both within the largest twenty-five denominations in the nation. Both religions have also developed significant international branches, organically and hierarchically linked to their American headquarters. Almost 9 million international members are reported by Jehovah’s Witnesses and 21 million for Seventh-day Adventists. Conservative growth projections for Seventh-day Adventists envisage that the number of worldwide adherents may approach 100 million by the mid-to-late 21st century. Both churches have impacted church–state relations in the United States through the legal system, seeking protection of civil rights and religious liberty. Seventh-day Adventism also has influenced American culture through its emphasis on health and education.