thumb|Take the Fair Face of Woman, and Gently Suspending, With Butterflies, Flowers, and Jewels Attending, Thus Your Fairy is Made of Most Beautiful Things, painting [[Sophie Gengembre Anderson]]
A Breton lai, also known as a narrative lay or simply a lay, is a form of medieval French and English romance literature. Lais are short (typically 600–1000 lines), rhymed tales of love and chivalry, often involving supernatural and fairy-world Celtic motifs. The word "lay" or "lai" is thought to be derived from the Old High German and/or Old Middle German leich, which means play, melody, or song, or possibly derived from the Irish word () meaning "song".
The earliest Arthurian lai to have survived is Robert Biket's , dating to the mid- to late-12th century.
Breton lais may have inspired Chrétien de Troyes, and likely were responsible for spreading Celtic and fairy-lore into Continental Europe. An example of a 14th-century Breton lai (the one later remade into the Middle English Sir Orfeo) has the king of the fairies carrying away a wife to the land of fairy.
- The Old Norse prose Möttuls saga and Middle High German verse are considered adaptations of the Lai du cort mantel, whose author may have been familiar with the Lai du cor.