Factions in The Elder Scrolls Online consist of the main Alliances, joinable Guilds, and other groups with similar goals, causes or philosophies.
During character customization, players must choose from one of three Alliances; the Aldmeri Dominion, the Daggerfall Covenant, and the Ebonheart Pact. Players are able to join various Guilds, regardless of their Alliance, and these include the Mages Guild, the Fighters Guild, and the Undaunted. The Thieves Guild and Dark Brotherhood will be added in-game post-launch. Other factions offer quests, but cannot be joined at launch, such as the Great Houses of Morrowind. While certain factions can only be encountered in a particular Alliance's territory, players may experience another Alliance's content when they reach level 50.
In 2E 578 an arcane explosion in the Imperial City set off a mystical aftershock that swept across Nirn. Many mages died or went mad. As the White-Gold Tower got into the hands of Molag Bal cultists, the border between the plane of Oblivion and Nirn was broken. The Daedra spawned all over Tamriel in greater numbers than ever before. The constellation of the Serpent grew so large that it dominated the night sky.
Without an Emperor sitting on the Ruby Throne, The Empire fell into disrepair. Facing a great danger, many provinces united into different factions, each seeking to reclaim the throne and fulfill their own agendas.
The main goal of the Aldmeri Dominion was to protect Tamriel from the Daedric disasters that Nirn faced, and spare the continent from the corruption of Men. To their minds, the short-lived race of Men were not worthy of ruler's role, and the only solution would be to take the throne for themselves.
The dominant race of the Daggerfall Covenant was the Breton. With the help of their merchant' skillful diplomacy, the Daggerfall Covenant manages to ally with the races of Redguard and Orsimer. The faction was led by the merchant High King Emeric.
With the fall of the Imperial City the renown trade routes of the Daggerfall Covenant races were disrupted.
With no more trade the region could not prosper. Poverty and suffering spread throughout the land. By reclaiming the Ruby Throne, the Daggerfall Covenant sought to return the Empire and, more importantly, cities of the Iliac Bay to their former wealth.
The dominant race of the Ebonheart Pact was the Nord. The Dunmer and, eventually, the Argonians joined forces along with the Nords to survive the Second Akaviri Invasion. The leader of the faction was High King Jorunn. He, however, did not have all the power over the provinces, as a Great Moot was summoned, a council of all the allied races, whom he had to hear out.
Having protected Tamriel from the Akaviri, the members of the Ebonheart Pact thought to have earned the right as the rulers. Moreover, ever since the great conquests of the Nords in the First Era, they thought themselves to be superior and the rightful rulers of Tamriel.
The Mages Guild appear in The Elder Scrolls Online. . One way to increase reputation with this faction is to search Tamriel for hidden Lore books.  Sufficient reputation increase will unlock additional skills, perks, and abilities. 
The Fighters Guild is a joinable faction in this game. One way to increase reputation in this guild is to destroy Dark Anchors of Molag Bal.  With enough reputation, new skills and abilities will be unlocked. 
The Undaunted is a guild involved in dungeons, and has its own unique skill line.
The Thieves Guild is dedicated to the gathering and training of those who are stealthy and shadowy in nature. Although criminal by its very definition, for untold years, local authorities in places throughout Tamriel have tolerated the existence of the guild for its role as "Crime Regulator"
The Dark Brotherhood is a guild of professional assassins which historically possessed considerable influence and power but has recently fallen on considerably harder times.
- A collection of fierce warriors who honored the ways of the original five hundred companions that arrived in Tamriel with their leader, the legendary Ysgramor. Once spread out throughout the provinces, a fraction of the once great army is what remains in Skyrim. The Companions are a well known and respected group, who deal with threats that endanger the lives of the commoners.
- House Telvanni is one of the six Great Houses of Morrowind. They are known as the wizard-lords who prefer to pursue wisdom and mastery in solitude. House Telvanni refused to join the Ebonheart Pact, causing inner conflict in the province of Morrowind.
- House Redoran is one of the Great Houses that have joined the Ebonheart Pact.
- The group that is responsible for attacking northern Valenwood.
- The current ruling family in Cyrodiil. Members include Clivia Tharn, the Empress Regent of Tamriel, and also her father, Elder Council member and Battlemage, Abnur Tharn. 
- Similar to the Forsworn group that exists in the Fourth Era. However at this point in time, they have allied themselves with Molag Bal's forces. . They are hostile to both Nords and Bretons. 
- They have continued animosity towards the Forebears, but are temporarily working together. 
- They have continued animosity towards the Crowns, but are temporarily working together. 
- A group of assassins trying to assassinate King Casmere in Daggerfall. 
- These may be the Bloodthorn. 
- A band of pirates that control the town of Saintsport in Stros M'kai and are lead by Captain Helane. 
For factions in other games or all the factions in The Elder Scrolls Universe see:
- Factions (Arena)
- Factions (Daggerfall)
- Factions (Morrowind)
- Factions (Oblivion)
- Factions (Skyrim)
- ↑ August 2, 2013. Twitch.TV QuakeCon, Day 2: The Elder Scrolls Online, John Carmack, and more!
- ↑ November 18, 2013. The Elder Scrolls Online Ask Us Anything: Variety Pack 11
- ↑ 3.0 3.1 Sep 3, 2013 Tamriel Foundry PAX Prime Interview - Paul Sage
- ↑ November 14, 2012. The Elder Scrolls Online Support Are the Mages Guild and Fighters Guild in the game?
- ↑ 5.0 5.1 5.2 5.3 Ask Us Anything: Ebonheart Pact (Part 2)
- ↑ August 9, 2013. The Elder Scrolls Online Support Will each faction be limited to specific areas, or can I travel anywhere?
- ↑ 7.0 7.1 7.2 The Elder Scrolls Online Story Guide
- ↑ 8.0 8.1 8.2 8.3 The Elder Scrolls Online Aldmeri Dominion Guide
- ↑ 9.0 9.1 9.2 The Elder Scrolls Online Daggerfall Covenant Guide
- ↑ 10.0 10.1 10.2 The Elder Scrolls Online Ebonheart Pact Guide
- ↑ 11.0 11.1 11.2 11.3 11.4 Game Informer - June 2012 Edition
- ↑ November 26, 2013. The Elder Scrolls Online Ask Us Anything: Ebonheart Pact (Part 2)
- ↑ December 20, 2012. The Elder Scrolls Online Ask Us Anything: Aldmeri Dominion Part 2
- ↑ The Elder Scrolls Online Interactive Map of Tamriel: Greenshade
- ↑ The Elder Scrolls Online Interactive Map of Tamriel: Reaper's March
- ↑ 16.0 16.1 January 14, 2013. The Elder Scrolls Online. Pledge Your Allegiance to the Daggerfall Covenant
- ↑ January 7, 2012. The Elder Scrolls Online Ask Us Anything: Daggerfall Covenant (Part 1)
- ↑ January 18, 2013. The Elder Scrolls Online (Lawrence Schick). Alliances at War
- ↑ 19.0 19.1 19.2 19.3 January 22, 2013. The Elder Scrolls Online. Ask Us Anything: Daggerfall Covenant Part 2
- ↑ July 2013. Kemps, Keidi.. PC Power Play, Issue #217.
- ↑ February 10, 2014. Capsule Computing The Elder Scrolls Online: Preview
- ↑ February 08, 2014. MMORPG Elder Scrolls Online Previews: Adventuring in Stros M’kai