Geoffrey of Monmouth's Historia Regum Britanniae (English: The History of the Kings of Britain) was written around 1136. It chronicles the lives of the kings of the Britons in a chronological narrative spanning a time of two thousand years. It begins with the Trojans in Homer's The Illiad and continues until the Anglo-Saxons had assumed control of Britain around the 7th century.

Of the many rulers mentioned in the history, most notable are:

• Brutus of Britain - Founded the colony of Britain and named the island after himself
• King Lear - Romanticized in Shakespeare's play King Lear
• Cassivelaunus - King of the Britons when Julius Caesar invaded
• Cymbeline - Parodied in Shakespear's tragicomedy Cymbeline
• Lucius of Britain - First Christian king in Britain
• Old King Cole - Romanticized in a nursery rhyme
• Constantine the Great - First Christian Roman Emperor
• Vortigern - Famous king written in many medieval stories
• King Arthur - Most famous of all legendary kings

The history of Geoffrey is rough and unreliable but forms the basis for much English lore and literature. The source of the history comes from Nennius and Gildas as well as Welsh chronicles and lost documents to which he refers but now seems lost. Most historians see the Historia as a work of fiction with some truth mixed within.

Geoffrey of Monmouth's narrative is one of the central pieces in the Matter of Britain.