Memory, Sorrow and Thorn is Tad Williams's epic fantasy trilogy, comprising The Dragonbone Chair (1988), Stone of Farewell (1990) and To Green Angel Tower (1993). For its paperback publication, To Green Angel Tower was divided into two volumes (called "Part 1" and "Part 2"), so the majority of Williams's readers will probably think Memory, Sorrow and Thorn more properly called a tetralogy than a trilogy.

The books are set in Osten Ard, peopled by humans, trolls (not giants, as in The Lord of the Rings and other fantasy series, but dwarf-sized) and Sithi - elflike people with catlike facial features and golden skin. There are several human countries, but they have been united under one rule by King John the Presbyter.

The story and the world in which it takes place draw extensively upon a plethora of sources from mediaeval European history and folklore. Several human nations populate Osten Ard, for instance, but most are direct parallels with European or Eurasian ethnic groups:

• Erkynland represents England, with the people of Warinsten and the Hayholt very faintly resembling Normans.
• Hernystir represents Ireland.
• The Rimmersmen represent the Vikings.
• The Nabbanai represent the ancient Romans and mediaeval Byzantines.
• The Thrithings folk represent the horse peoples of the Steppe, such as the Magyars, Khazars and Mongols.

Additionally, most humans are followers of a reinvented, fantasy version of Christianity known as Aedonism, whose Christ-like figure Usires Aedon, called Ransomer rather than Messiah, was hanged on a tree rather than nailed to a cross.

Several of the characters are also given overtones of people from British lore or history: Prester John, for instance, is compared to King Arthur, Sir Camaris is compared to Sir Lancelot and Prince Josua is likened to Alfred the Great.

Williams is able to use these allusions throughout the trilogy, alternately to give Osten Ard added depth and complexity that would not be possible with a wholly invented milieu, and to set up expectations in the reader that he can then either gratify or confound.