These olde gentil Britouns in hir dayes
Of diverse aventures maden layes,
Rymeyed in hir firste Briton tonge,
Whiche layes with hir instrumentz they songe
Or elles redden hem for hir plesaunce
(Franklin's Prologue. V.709-10)

The "Breton lays" are short romances, often (but not always) based on the earlier French lais of Marie de France. Most often they involve love and the supernatural; Chaucer calls his Franklin's Tale a "Breton lay" but it is a very unusual example of the genre.

The earliest Breton lays were written by Marie de France, who lived and wrote in England in the twelfth century. See the home page of the International Marie de France Society. The page includes translations of all of Marie's lays; they are well worth browsing. A celebrated fourteenth-century English definition of the Breton lay is in:

The Prologue to the Lay de Frein

Of the samples listed here, the Earl of Toulouse and Emaré are labelled "Breton lays" by their authors, but they are really merely brief romances that invoke the concept of the Breton lay as a means of establishing the authority of their tales (but they are good reading nevertheless):

The Earl of Toulouse
Emaré

The lay of Sir Launfal is much more clearly in the tradition of the Breton lay as it is usually understood and it is one of the best.

Sir Launfal

Compare Marie's Lanval (the ultimate source of Sir Launfal), an annotated verse translation by Judith P. Shoaf, available on the Exemplaria web site.

One of the best of the English lays is: Sir Orfeo This is translated into modern English by Jessie L.Weston (it's a bit old-fashioned but fun to read).

The texts cited above have been regularized and slightly emended. They are intended for beginning students; for texts nearer the MSS, well annotated, see Anne Laskaya and Eve Salisbury. (Kalamazoo, Michigan: Western Michigan University for TEAMS, 1995). These are available on the Web from the TEAMS page.

See also:

Donovan, Mortimer J.. The Breton Lay: a Guide to Varieties. Notre Dame [Ind.] University of Notre Dame Press [1969] [PQ207.D6].

Marie de France. The Lais of Marie de France. Penguin classics. Harmondsworth, Middlesex, England; New York, N.Y., U.S.A.: Penguin Books, 1986 [PQ1494.L3 E5 1986].

Rumble, Thomas C. ed. The Breton Lays in Middle English. Detroit: Wayne State University Press, 1965 [Widener: 27271.56].

Rickert, Edith. Early English Romances in Verse. The New Medieval Library.