Great House

Regions_of_Westeros.PNG:

House Stark, the North (grey);

House Arryn, the Vale (blue);

House Tully, the Riverlands (green);

House Greyjoy, the Iron Islands (yellow);

House Lannister, the Westerlands (red);

House Tyrell, the Reach (lime green);

House Baratheon, the Stormlands (brown);

House Martell, Dorne (violet).

The Crownlands (pink) are ruled directly from the Iron Throne; having previously been the lands of House Targaryen.

Wildling territories are shown in black.]] The Great Houses of Westeros are the most powerful of the Noble houses in the Seven Kingdoms. They exercise immense authority and power over their vassal houses and the areas that control, and are typically answerable only to the King on the Iron Throne.

The Great Houses

 * House Arryn, rulers of the Vale of Arryn from the castle at the Eyrie.
 * House Baratheon, who currently hold the Iron Throne and also rule the Stormlands from Storm's End.
 * House Greyjoy, rulers of the Iron Islands from the castle at Pyke.
 * House Lannister, rulers of the Westerlands from the castle at Casterly Rock.
 * House Martell, rulers of Dorne from the castle at Sunspear.
 * House Stark, rulers of the North from the castle at Winterfell.
 * House Tully, rulers of the Riverlands from the castle at Riverrun.
 * House Tyrell, rulers of the Reach from the castle at Highgarden.
 * House Targaryen, the former royal house, displaced during Robert's Rebellion which brought House Baratheon to power.

In the books
Keeping in mind that author George R.R. Martin has said that Westeros is roughly the size of the continent of South America, the realm known as "the Seven Kingdoms" spans an area on the scale of the Roman Empire. Thus in terms of their populations and the size of the armies they can raise in wartime, each of the "Great Houses" is more comparable in scale to a real-life medieval kingship like England or France than a smaller dukedom.