Adventism is a Christian movement which began in the 19th century, in the context of the Second Great Awakening revival in the United States. The name refers to belief in the imminent Second Coming (or "Second Advent") of Jesus Christ. It was started by William Miller, whose followers became known as Millerites. Today, the largest church within the movement is the Seventh-day Adventist Church.

The Adventist family of churches is regarded today as conservative Protestants. Although these churches hold much in common, their theology differs on whether the intermediate state is unconscious sleep or consciousness, whether the ultimate punishment of the wicked is annihilation or eternal torment, the nature of immortality, whether or not the wicked are resurrected after the millennium, and whether the sanctuary of Daniel 8 refers to the one in heaven or one on earth. The movement has encouraged the examination of the New Testament, leading them to observe the Sabbath.

Adventism began as an inter-denominational movement. Its most vocal leader was William Miller. Between 50,000 and 100,000 people in the United States supported Miller's predictions of Christ's return. After the "Great Disappointment" of October 22, 1844 many people in the movement gave up on Adventism. Of those remaining Adventist, the majority gave up believing in any prophetic (biblical) significance for the October 22 date, yet they remained expectant of the near Advent (second coming of Jesus). Of those who retained the October 22 date, many maintained that Jesus had come not literally but "spiritually", and consequently were known as "spiritualizers". A small minority held that something concrete had indeed happened on October 22, but this event had been misinterpreted. This viewpoint later emerged and crystallized with the Seventh-day Adventist Church, the largest remaining body today.