The Great Houses are the most powerful of the noble Houses of the Seven Kingdoms. They exercise immense authority and power over their vassals and territories and are answerable only to the King of the Andals and the First Men, with the exception of House Stark, which rules over an independent kingdom.

Before the Targaryen Conquest, each of the Seven Kingdoms was ruled independently by a powerful royal family. During the Conquest these families were either destroyed, replaced, or defeated and made to swear allegiance to King Aegon Targaryen, who reigned from the Iron Throne in King's Landing.

The Gardeners, Hoares, and Durrandons were slain and replaced by the Tyrells, Greyjoys, and Baratheons respectively, while the Lannisters, Arryns, and Starks surrendered peacefully and were allowed to remain in control of their lands. In addition, Aegon raised up the Tullys to rule over the Riverlands, which had been under Ironborn occupation prior to his arrival. Only House Martell of Dorne resisted his armies, but was later brought into the fold through a peaceful marriage-alliance some two centuries later.

Each Great House has a large number of vassal houses serving it, most of whom in turn have smaller vassals under them, extending all the way down to farmers and landed knights. Each Great House rules over its territory and is responsible for collecting taxes and, in times of war, raising troops to fight for the king. Otherwise they are largely left to operate autonomously to simplify the bureaucracy and governance of the realm.

Robert's Rebellion ended in the exile of the last Targaryens, with House Baratheon replacing them on the Iron Throne, forming the royal cadet branch House Baratheon of King's Landing.

The War of the Five Kings, along with Daenerys Targaryen's war for Westeros, has seen changes amongst the Great Houses. House Baratheon is no longer the royal house, and two of it's branches have become extinct, House Tyrell is also extinct, and has been succeeded by Bronn's unnamed House. The Riverlands and the titles of House Tully, which is also near extinction, defeated in the war and exiled, have been given to its usurping former vassals House Frey, though the other Tully vassals remain opposed to the Freys, whose male line has, meanwhile, been effectively wiped out, allowing the Tullys to reclaim their position. With the coronation of Queen Cersei Lannister, the Crownlands was held by House Lannister, the new royal house after the legal extinction of the Baratheons (though after the legitimization of Gendry Baratheon by Queen Daenerys Targaryen, House Baratheon has been restored). Although scattered early in the war and stripped of lands and titles, House Stark has regained control of the North through the defeat of its usurping vassals House Bolton, and later gained independence. After the battle of King's Landing, the Lannisters' brief reign as Royal House ended, and once again, they only rule the Westerlands. However, the Lannisters face an uncertain future, with all but one of them dead, and their gold mines depleted.

*For more about the Great Houses of Westeros, visit here.