Tristan and Iseult is a romance story, retold in numerous sources with as many variations since the 12th century. The story is a tragedy about the adulterous love between the Cornish knight Tristan (Tristram, etc.) and the Irish princess Iseult (Isolde, Yseult, etc.). The narrative predates and most likely influenced the Arthurian romance of Lancelot and Guinevere, and has had a substantial impact on Western art and literature. While the details of the story differ from one author to another, the overall plot structure remains much the same.
The story and character of Tristan vary from author to author; even the spelling of his name varies a great deal, although "Tristan" is the most popular spelling. Nevertheless, there are two main traditions of the Tristan legend. The early tradition comprised the French romances of Thomas of Britain and Béroul, two poets from the second half of the 12th century. Later traditions come from the vast Prose Tristan (c. 1240), which was markedly different from the earlier tales written by Thomas and Béroul.
After defeating the Irish knight Morholt, Tristan travels to Ireland to bring back the fair Iseult (also appearing under various spellings) for his uncle, King Mark of Cornwall, to marry. Along the way, they ingest a love potion which causes the pair to fall madly in love. In the courtly version, the potion's effects last a lifetime, but, in the common versions, the potion's effects wane after three years. In some versions, they ingest the potion accidentally; in others, the potion's maker instructs Iseult to share it with Mark, but she deliberately gives it to Tristan instead. Although Iseult marries Mark, she and Tristan are forced by the potion to seek one another, as lovers. While the typical noble Arthurian character would be shamed by such an act, the love potion that controls them frees Tristan and Iseult from responsibility. The king's advisors repeatedly endeavour to have the pair tried for adultery, but the couple continually use trickery to preserve their façade of innocence. In Béroul's version, the love potion eventually wears off, and the two lovers are free to make their own choice as to whether to cease their adulterous relationship or to continue.
As with the Arthur-Lancelot-Guinevere love triangle in the medieval courtly love motif, Tristan, King Mark, and Iseult of Ireland all love each other. Tristan honours and respects King Mark as his mentor and adopted father; Iseult is grateful that Mark is kind to her; and Mark loves Tristan as his son and Iseult as a wife. But every night, each has horrible dreams about the future. Tristan's uncle eventually learns of the affair and seeks to entrap his nephew and his bride. Also present is the endangerment of a fragile kingdom, the cessation of war between Ireland and Cornwall (Dumnonia). Mark acquires what seems proof of their guilt and resolves to punish them: Tristan by hanging and Iseult by burning at the stake, later lodging her in a leper colony. Tristan escapes on his way to the gallows. He makes a miraculous leap from a chapel and rescues Iseult. The lovers escape into the forest of Morrois and take shelter there until discovered by Mark. They make peace with Mark after Tristan's agreement to return Iseult of Ireland to Mark and leave the country. Tristan then travels to Brittany, where he marries (for her name and her beauty) Iseult of the White Hands, daughter of Hoel of Brittany and sister of Kahedin.
The earliest surviving versions already incorporate references to King Arthur and his court. The connection between Tristan and Iseult and the Arthurian legend was expanded over time, and sometime shortly after the completion of the Vulgate Cycle (the Lancelot-Grail) in the first quarter of the 13th century, two authors created the Prose Tristan, which fully establishes Tristan as a Knight of the Round Table who even participates in the Quest for the Holy Grail. The Prose Tristan became the common medieval tale of Tristan and Iseult that would provide the background for Thomas Malory, the English author who wrote Le Morte d'Arthur over two centuries later.
In the Prose Tristan and works derived from it, Tristan is mortally wounded by King Mark, who strikes Tristan with a lance from Morgan le Fay while Tristan is playing a harp for Iseult. The poetic versions of the Tristan legend offer a very different account of the hero's death. According to Thomas' version, Tristan was wounded by a poison lance while attempting to rescue a young woman from six knights. Tristan sends his friend Kahedin to find Iseult of Ireland, the only person who can heal him. Tristan tells Kahedin to sail back with white sails if he is bringing Iseult, and black sails if he is not. Iseult agrees to return to Tristan with Kahedin, but Tristan's jealous wife, Iseult of the White Hands, lies to Tristan about the colour of the sails. Tristan dies of grief, thinking that Iseult has betrayed him, and Iseult dies swooning over his corpse. Several versions of the Prose Tristan include the traditional account of Tristan's death found in the poetic versions.
In French sources, such as those picked over in the English translation by Hilaire Belloc in 1903, it is stated that a thick bramble briar grows out of Tristan's grave, growing so much that it forms a bower and roots itself into Iseult's grave. It goes on that King Mark tries to have the branches cut three separate times, and each time the branches grow back and intertwine. This behaviour of briars would have been very familiar to medieval people who worked on the land. Later tellings sweeten this aspect of the story, by having Tristan's grave grow a briar, but Iseult's grave grow a rose tree, which then intertwine with each other. Further variants refine this aspect even more, with the two plants being said to have been hazel and honeysuckle.
A few later stories even record that the lovers had a number of children. In some stories they produced a son and a daughter they named after themselves; these children survived their parents and had adventures of their own. In the French romance Ysaie le Triste (Ysaie the Sad), the eponymous hero is the son of Tristan and Iseult; he becomes involved with the fairy king Oberon and marries a girl named Martha, who bears him a son named Mark. Spanish Tristan el Joven also dealt with Tristan's son, here named Tristan of Leonis.
There are many theories present about the origins of Tristanian legend, but historians disagree over which is the most accurate.
The mid-6th-century Drustanus Stone monument in Cornwall has an inscription seemingly referring to Drustan, son of Cunomorus ("Mark"). However, not all historians agree that the Drustan referred to is the archetype of Tristan.
There are references to March ap Meichion ("Mark") and Trystan in the Welsh Triads, in some of the gnomic poetry, the Mabinogion stories, and in the 11th-century hagiography of Illtud. A character called Drystan appears as one of King Arthur's advisers at the end of The Dream of Rhonabwy, an early 13th-century tale in the Welsh prose collection known as the Mabinogion. Iseult is listed along with other great men and women of Arthur's court in another, much earlier Mabinogion tale, Culhwch and Olwen.
Possible Irish antecedents to the Tristan legend have received much scholarly attention. An ill-fated triantán an ghrá or love triangle features into a number of Irish works, most notably in the text called Tóraigheacht Dhiarmada agus Ghráinne or The Pursuit of Diarmuid and Gráinne. In the story, the aging Fionn mac Cumhaill takes the young princess, Gráinne, to be his wife. At the betrothal ceremony, however, she falls in love with Diarmuid, one of Fionn's most trusted warriors. Gráinne gives a sleeping potion to all present but him, eventually convincing him to elope with her. The fugitive lovers are then pursued all over Ireland by the Fianna.
Another Irish analogue is Scéla Cano meic Gartnáin, preserved in the 14th-century Yellow Book of Lecan. In this tale, Cano is an exiled Scottish king who accepts the hospitality of King Marcan of Ui Maile. His young wife, Credd, drugs all present, and then convinces Cano to be her lover. They try to keep a tryst while at Marcan's court, but are frustrated by courtiers. Eventually Credd kills herself and Cano dies of grief.
In the Ulster Cycle there is the text Clann Uisnigh or Deirdre of the Sorrows in which Naoise mac Usnech falls for Deirdre, who was imprisoned by King Conchobar mac Nessa due to a prophecy that Ulster would plunge into civil war due to men fighting for her beauty. Conchobar had pledged to marry Deirdre himself in time to avert war, and takes his revenge on Clann Uisnigh. The death of Naoise and his kin leads many Ulstermen to defect to Connacht, including Conchobar's stepfather and trusted ally Fergus mac Róich, eventually precipitating the Táin Bó Cúailnge.
Some scholars suggest that the 11th-century Persian story Vis and R?min must have been the model for the Tristan legend because the similarities are too great to be coincidental. The evidence for the Persian origin of Tristan and Iseult is very circumstantial and different theories have been suggested how this Persian story might have reached the West, some suggesting story-telling exchanges during the crusades in Syrian court and through minstrels who had free access to both Crusader and Saracen camps in the Holy Land.
Some believe Ovid's Pyramus and Thisbe, as well as the story of Ariadne at Naxos might have also contributed to the development of the Tristan legend. The sequence in which Tristan and Iseult die and become interwoven trees also parallels Ovid's love story of Baucis and Philemon in which two lovers are transformed in death into two different trees sprouting from the same trunk. However this also occurs in the saga of Deidre of the Sorrows making the link more tenuous and ignores the (now lost) oral traditions of preliterate societies, relying only on written records which are known to have been damaged - especially during the Dissolution of the Monasteries - during the development of modern nation states such as England and France.
The earliest representation of what scholars name the "courtly" version of the Tristan legend is in the work of Thomas of Britain, dating from 1173. Only ten fragments of his Tristan poem, representing six manuscripts, have ever been located: the manuscripts in Turin and Strassburg are now lost, leaving two in Oxford, one in Cambridge and one in Carlisle. In his text, Thomas names another trouvère who also sang of Tristan, though no manuscripts of this earlier version have been discovered. There is also a passage telling how Iseult wrote a short lai out of grief that sheds light on the development of an unrelated legend concerning the death of a prominent troubadour, as well as the composition of lais by noblewomen of the 12th century.
The next essential text for knowledge of the courtly branch of the Tristan legend is the abridged translation of Thomas made by Brother Robert at the request of King Haakon Haakonson of Norway in 1227. King Haakon had wanted to promote Angevin-Norman culture at his court, and so commissioned the translation of several French Arthurian works. The Nordic version presents a complete, direct narrative of the events in Thomas' Tristan, with the telling omission of his numerous interpretive diversions. It is the only complete representative of the courtly branch in its formative period.
Preceding the work of Brother Robert chronologically is the Tristan and Isolt of Gottfried von Strassburg, written circa 1211-1215. The poem was Gottfried's only known work, and was left incomplete due to his death with the retelling reaching half-way through the main plot. The poem was later completed by authors such as Heinrich von Freiberg and Ulrich von Türheim, but with the "common" branch of the legend as the ideal source.
The earliest representation of the "common branch" is Béroul's Le Roman de Tristan, the first part of which is generally dated between 1150 and 1170, and the latter part between 1181 and 1190. The branch is so named due to its representation of an earlier non-chivalric, non-courtly, tradition of story-telling, making it more reflective of the Dark Ages than of the refined High Middle Ages. In this respect, they are similar to Layamon's Brut and the Perlesvaus. As with Thomas' works, knowledge of Béroul's is limited. There were a few substantial fragments of his works discovered in the 19th century, and the rest was reconstructed from later versions. This being said, Beroul's version is the oldest known version of the Tristan romances and is commonly considered to come the closest to presenting all of the raw events in the romance exactly as they are, with no explanation or modifications. Therefore, Beroul's version is an archetype for later "common branch" editions. 
One aspect of the common branches that differentiates them significantly from the courtly branches is their depiction of the lovers' time in exile from Mark's court. While the courtly branches describe Tristan and Iseult as sheltering in a "Cave of Lovers" and living in happy seclusion, thus keeping with the tradition of courtly and chivalric writing, the common branches emphasize the extreme suffering that Tristan and Iseult endure. In the common branches, the exile is a true punishment that highlights the couple's departure from courtly norms and emphasizes the impossibility of their romance. 
The French medievalist Joseph Bédier thought all the Tristan legends could be traced to a single original poem, adapted by Thomas of Brittany into French from an original Cornish or Breton source. He dubbed this hypothetical original the "Ur-Tristan", and wrote his still-popular Romance of Tristan and Iseult as an attempt to reconstruct what this might have been like. In all likelihood, common branch versions reflect an earlier form of the story; accordingly, Bédier relied heavily on Eilhart, Béroul and Gottfried von Strassburg, and incorporated material from other versions to make a cohesive whole. A new English translation of Bédier's Roman de Tristan et Iseut (1900) by Edward J. Gallagher was published in 2013 by Hackett Publishing Company. A translation by Hilaire Belloc, first published in 1913, it was published in 1958 as a Caedmon Audio recording read by Claire Bloom and republished in 2005.
Contemporary with Béroul and Thomas, Marie de France presented a Tristan episode in one of her lais: "Chevrefoil". It concerns another of Tristan's clandestine returns to Cornwall in which the banished hero signals his presence to Iseult by means of an inscription on a branch of a hazelnut tree placed on the road she will travel. The title refers to the symbiosis of the honeysuckle and hazelnut tree which die when separated, as do Tristan and Iseult: "Ni vous sans moi, ni moi sans vous." ("Neither you without me, nor me without you.") This episode is reminiscent of one in the courtly branch when Tristan uses wood shavings put in a stream as signals to meet in the garden of Mark's palace.
There are also two 12th-century Folies Tristan, Old French poems identified as the Berne and the Oxford versions, which relate Tristan's return to Marc's court under the guise of a madman. Besides their own importance as episodic additions to the Tristan story and masterpieces of narrative structure, these relatively short poems significantly contributed to restoring the missing parts of Béroul's and Thomas' incomplete texts.
Chrétien de Troyes claims to have written a Tristan story, though no part of it has ever been found. He mentions this in the introduction to Cligès, a romance that many see as a kind of anti-Tristan with a happy ending. Some scholars speculate his Tristan was ill-received, prompting Chretien to write Cligès - a story with no Celtic antecedent - to make amends.
After Béroul and Thomas, the most important development in French Tristaniana is a complex grouping of texts known broadly as the Prose Tristan. Extremely popular in the 13th and 14th century, the narratives of these lengthy versions vary in detail from manuscript to manuscript. Modern editions run twelve volumes for the long version, which includes Tristan's participation in the Quest for the Holy Grail, or five volumes for a shorter version without the Grail Quest. It had a great influence on later medieval literature, and inspired parts of the Post-Vulgate Cycle and the Roman de Palamedes.
The earliest complete source of the Tristan material in English was Sir Tristrem, a romance of some 3344 lines written circa 1300. It is preserved in the famous Auchinleck manuscript at the National Library of Scotland. The narrative largely follows the courtly tradition. As is true with many medieval English adaptations of French Arthuriana, the poem's artistic achievement can only be described as average, though some critics have tried to rehabilitate it, claiming it is a parody. Its first editor, Walter Scott, provided a sixty line ending to the story, which has been printed with the romance in every subsequent edition.
The only other medieval handling of the Tristan legend in English is Thomas Malory's The Book of Sir Tristram de Lyones, a shortened "translation" of the French Prose Tristan, in his Le Morte d'Arthur. Since the Winchester Manuscript surfaced in 1934, there has been much scholarly debate whether the Tristan narrative, like all the episodes in Le Morte d'Arthur, was originally intended to be an independent piece or part of a larger work.
The popularity of Brother Robert's version spawned a unique parody, Saga Af Tristram ok Ísodd, as well as the poem Tristrams kvæði. In the collection of Old Norse prose-translations of Marie de France's lais - called Strengleikar (Stringed Instruments) - two lais with Arthurian content have been preserved, one of them being the "Chevrefoil", translated as "Geitarlauf".
By the 19th century, scholars had found Tristan legends spread across the Nordic world, from Denmark to the Faroe Islands. These stories, however, diverged greatly from their medieval precursors. In one Danish ballad, for instance, Tristan and Iseult are made brother and sister. Other unlikely innovations occur in two popular Danish chapbooks of the late 18th-century Tristans saga ok Inionu and En tragoedisk Historie om den ædle og tappre Tistrand, in which Iseult is made the princess of India. The popularity of these chapbooks inspired Icelandic poets Sigurður Breiðfjörð and Níels Jónsson to write rímur, long verse narratives, inspired by the Tristan legend.
A 158-line fragment of a Dutch version (ca. 1250) of Thomas of Britain's Tristan exists. It is being kept in the Austrian National Library in Vienna, Series nova 3968. A short Tristan narrative, perhaps related to the Béroul text, exists in six Welsh manuscripts dating from the late 16th to the mid 17th century.
In the first third of the 14th century, Arcipreste de Hita wrote his version of the Tristan story, Carta enviada por Hiseo la Brunda a Tristán. Respuesta de Tristán was a unique 15th-century romance written in the form of imaginary letters between the two lovers. Libro del muy esforzado caballero Don Tristán de Leonís y de sus grandes hechos en armas, Spanish reworking of the Prose Tristan, was first published in Valladolid in 1501.
In Italy, there were many cantari, or oral poems performed in the public square, either about Tristan or frequently referencing him, including Cantari di Tristano; Due Tristani; Quando Tristano e Lancielotto combattiero al petrone di Merlino; Ultime imprese e morte Tristano; and Vendetta che fe Messer Lanzelloto de la Morte di Messer Tristano.
There are also four differing versions of the Prose Tristan in medieval Italy, most named after their place of composition or library in which they are currently to be found: Tavola Ritonda, Tristano Panciaticchiano, Tristano Riccardiano, and Tristano Veneto.
A 13th-century verse romance exists in Czech, based on the German Tristan poems by Gottfried von Strassburg, Heinrich von Freiberg and Eilhart von Oberge. It is the only known verse representative of the Tristan story in a Slavic language.
The Belarusian prose Povest o Tryshchane represents the furthest eastern advance of the legend, and, composed in the 1560s, is considered by some critics to be the last "medieval" Tristan or Arthurian text period. Its lineage goes back to the Tristano Veneto. Venice, at that time, controlled large parts of the Serbo-Croatian language area, engendering a more active literary and cultural life there than in most of the Balkans during this period. The manuscript of the Povest states that it was translated from a (lost) Serbian intermediary. Scholars assume that the legend must have journeyed from Venice, through its Balkan colonies, finally reaching a last outpost in this Slavic language.
The Tristan story was represented in several art media, from ivory mirror-cases to the 13th-century Sicilian Tristan Quilt. Many of the manuscripts with literary versions are illuminated with miniatures.
In English, the Tristan story suffered the same fate as the Matter of Britain generally. After being mostly ignored for about three centuries, there was a renaissance of original Arthurian literature, mostly narrative verse, in the late 19th and early 20th centuries.
Tristan material in this revival included Alfred Tennyson's The Last Tournament, one of his Idylls of the King; Matthew Arnold's Tristram and Iseult. Another Victorian work was Algernon Charles Swinburne's epic poem Tristram of Lyonesse.
Thomas Hardy's The Famous Tragedy of the Queen of Cornwall at Tintagel in Lyonnesse is a one-act play which was published in 1923 (the book includes an imaginary drawing of the castle at the period).Rutland Boughton's opera The Queen of Cornwall (1924) was based on Thomas Hardy's play.
After World War II, most Tristan texts were in the form of prose novels or short stories:
In 1832, Gaetano Donizetti references this story in his opera L'elisir d'amore as the character of Adina sings the story to the ensemble, inspiring Nemorino to ask the charlatan Dulcamara for the magic elixir.
Twentieth-century composers also used the legend (often with Wagnerian overtones) in their compositions. Olivier Messiaen built his Turangalîla-Symphonie around the story. Hans Werner Henze's Tristan borrowed freely from the Wagnerian version as well as retellings of the legend.
The story has also been adapted into film many times.