The term Adventist generally refers to someone who believes in the Second Advent of Jesus (popularly known as the Second coming) in the tradition of the Millerites. The largest church within the movement today is the Seventh-day Adventist Church. The Adventist family of churches are regarded today as conservative Protestants. 
While they 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, and whether the sanctuary of Daniel 8 refers to the one in heaven or on earth. The movement has encouraged the examination of the Old Testament, leading some to observe the Sabbath and others to use the name "Jehovah" for God.
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 and presumably on Christianity as well. 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.
The Albany Conference in 1845, attended by 61 delegates, was called to attempt to determine the future course and meaning of the Millerite movement. Following this meeting, the "Millerites" then became known as "Adventists" or "Second Adventists". However, the delegates disagreed on several theological points. Four groups emerged from the conference: The Evangelical Adventists, The Life and Advent Union, the Advent Christian Church, and the Seventh-day Adventist Church.
The largest group organized as the American Millennial Association, a portion of which was later known as the Evangelical Adventist Church. Unique among the Adventists, they believed in an eternal hell and consciousness in death. They declined in numbers, and by 1916 their name did not appear in the United States Census of Religious Bodies. It has diminished to almost non-existence today. Their main publication was the Advent Herald. (Millerite magazine Signs of the Times) was first published in 1840. In January 1844 it was continued and renamed the Advent Herald and Signs of the Times or Advent Herald, of which Sylvester Bliss was the editor until his death in 1863. It was later called the Messiah’s Herald). Another was the Signs of the Times
The Life and Advent Union was founded by George Storrs in 1863. He had established The Bible Examiner in 1842. It merged with the Adventist Christian Church in 1964.
The Advent Christian Church officially formed in 1861, and grew rapidly at first. It declined a little over the 20th century. The Advent Christians publish the four magazines The Advent Christian Witness, Advent Christian News, Advent Christian Missions andMaranatha. They also operate a liberal arts college at Aurora, Illinois; and a Bible College at Lennox, Massachusetts. The Primitive Advent Christian Church later separated from a few congregations in West Virginia.
The Seventh-day Adventist Church officially formed in 1863. It believes in the sanctity of the seventh-day Sabbath as a holy day for worship. It published the Adventist Review and Sabbath Herald. It has grown to a large worldwide denomination and has a significant network of medical and educational institutions.
Miller did not join any of the movements, and spent the last few years of his life working for unity, before dying in 1849.
The Handbook of Denominations in the United States, 12th edn., describes the following churches as "Adventist and Sabbatarian (Hebraic) Churches":
- Christadelphians - The Christadelphians, were founded 1844 and had an estimated 25,000 members in 170 ecclesias in 2000 in America.
- Advent Christian Church - The Advent Christian Church was founded 1860 and had 25,277 members in 302 churches in 2002 in America. It is a "first-day" body of Adventist Christians founded on the teachings of William Miller. They adopted the "conditional immortality" views of Charles F. Hudson and George Storrs formed the "Advent Christian Association" in Salem, Massachusetts in 1860.
- Primitive Advent Christian Church - The Primitive Advent Christian Church is a small group which separated from the Advent Christian Church. They differ from the parent body mainly in two points. They observe feet washing as a rite of the church, and they teach that reclaimed backsliders should be baptized (even though they had formerly been baptized). This is sometimes referred to as rebaptism.
- Seventh-day Adventist - The Seventh-day Adventist Church, founded 1863, and had 15,600,000 baptized members (not counting children of members) worldwide as of 2007 is best known for its teaching that Saturday, the seventh day of the week, is the Sabbath and is the appropriate day for worship.
- Seventh Day Adventist Reform Movement - The Seventh Day Adventist Reform Movement is a small offshoot with an unknown number of members from the Seventh-day Adventist Church caused by disagreement over military service on the Sabbath day during World War I.
- Davidian Seventh-day Adventist Association - The Davidians (originally named Shepherd's Rod) is a small offshoot with an unknown number of members made up primarily of voluntarily disfellowshipped members of the Seventh-day Adventist Church. They were originally known as the Shepherd's Rod and are still referred to as such. The group derives its name from two books on Bible doctrine written by their founder, Victor Houteff, in 1929.
- Branch Davidians - The Branch Davidians were a split ("branch") from the Davidians. Many of them perished in the infamous Waco Siege of 1993.
- Church of God (Seventh Day) - The Church of God (Seventh-Day) founded 1863 with an estimated 11,000 members in 185 churches in 1999 in America. They separated in 1858 from those Adventists associated with Ellen White who later organized as Seventh-day Adventists in 1863. The Worldwide Church of God splintered from this. The Church of God (7th Day) split off in 1933.
- Church of God and Saints of Christ - The Church of God and Saints of Christ was founded 1896 and had an estimated 40,000 members in approximately 200 congregations in 1999 in America.
- Church of God General Conference - The Church of God General Conference was founded in 1921 and had 7,634 members in 162 churches in 2004 in America. It is an Adventist Christian body which is also known as the Church of God of the Abrahamic Faith and the Church of God General Conference (Morrow, GA).
- United Seventh-Day Brethren - The United Seventh-Day Brethren is a small Sabbatarian Adventist body. In 1947, several individuals and two independent congregations within the Church of God Adventist movement formed the United Seventh-Day Brethren, seeking to increase fellowship and to combine their efforts in evangelism, publications, and other ministries.
- Worldwide Church of God - The Worldwide Church of God was founded 1933 and had an estimated 63,000 members worldwide in 2004.
- United Church of God - Following massive doctrinal changes in the Worldwide Church of God, numerous groups split off to retain a more traditional system. The United Church of God founded in 1995 is the largest such offshoot.
Other minor Adventist groups
- True and Free Adventists, a Russian group 
- At least two denominations and numerous individual churches with a charismatic or Pentecostal-type bent have been influenced by or were offshoots from Seventh-day Adventists
- United Sabbath-Day Adventist Church, an African-American offshoot of the Seventh-day Adventist Church in New York City
- Celestia, a Christian communal town near Laporte in Sullivan County, Pennsylvania, founded by Millerite Peter E. Armstrong. It disintegrated before the end of the 19th century 
Jehovah's Witnesses are generally not regarded as part of the Millerite Adventist movement, although at least one source does.  The founder Charles Taze Russell attended an Adventist church 1870–4 and "was deeply influenced by Adventist thought".  Jehovah's Witnesses, previously known as International Bible Students before 1931, founded circa 1870, as of December 2008 there were 7 million Witnesses in 235 different lands attending 105,000 congregations.
- "Adventist and Sabbatarian (Hebraic) Churches" section (p. 256–276) in Frank S. Mead, Samuel S. Hill and Craig D. Atwood, Handbook of Denominations in the United States, 12th edn. Nashville: Abingdon Press
- George Knight, A Brief History of Seventh-day Adventists
- http://news.adventist.org/data/2007/09/1192400552/index.html.en" rel="nofollow">Church reports largest membership growth rate since 2002 audit
- Tarling, Lowell R. (1981). "The Churches of God". The Edges of Seventh-day Adventism: A Study of Separatist Groups Emerging from the Seventh-day Adventist Church (1844–1980). ="/w/index.php?title=Barragga_Bay,_New_South_Wales&action=edit&redlink=1" title="Barragga Bay, New South Wales (page does not exist)">Barragga Bay,="/wiki/Bermagui,_New_South_Wales" title="Bermagui, New South Wales">Bermagui South, ="/wiki/New_South_Wales" title="New South Wales">NSW: Galilee Publications. pp. 24–40. ="/wiki/International_Standard_Book_Number" title="International Standard Book Number">ISBN ="/wiki/Special:BookSources/0_9593457_0_1" title="Special:BookSources/0 9593457 0 1">0 9593457 0 1.
- http://www.spectrummagazine.org/files/archive/archive16-20/19-2alexeyeva.pdf" rel="nofollow">Human Rights and the True and Free Adventists" by Ludmilla Alexeyeva. ="/wiki/Spectrum_(magazine)" title="Spectrum (magazine)">Spectrum 19:2 (November 1988)
- Gary Land, Historical Dictionary of Seventh-day Adventists, p254
- http://h0bbes.wordpress.com/2006/10/13/celestia/" rel="nofollow">Celestia" blog by Jeff Crocombe, October 13, 2006
- Handbook, p268
- Tarling, Lowell R. (1981). The Edges of Seventh-day Adventism: A Study of Separatist Groups Emerging from the Seventh-day Adventist Church (1844–1980). ="/w/index.php?title=Barragga_Bay,_New_South_Wales&action=edit&redlink=1" title="Barragga Bay, New South Wales (page does not exist)">Barragga Bay, ="/wiki/Bermagui,_New_South_Wales" title="Bermagui, New South Wales">Bermagui South, ="/wiki/New_South_Wales" title="New South Wales">New South Wales: Galilee Publications. pp. 81. ="/wiki/International_Standard_Book_Number" title="International Standard Book Number">ISBN ="/wiki/Special:BookSources/0_9593457_0_1" title="Special:BookSources/0 9593457 0 1">0 9593457 0 1.