Mummers' plays are folk plays performed by troupes of amateur actors, traditionally all male, known as mummers or guisers (also by local names such as rhymers, pace-eggers, soulers, tipteerers, wrenboys, and galoshins). Historically, mummers' plays consisted of informal groups of costumed community members that visited from house to house on various holidays. The modern term refers especially to a play in which a number of characters are called on stage, two of whom engage in a combat, the loser being revived by a doctor character. This play is sometimes found associated with a sword dance though both also exist in Britain independently.

Plays may be performed in the street or during visits to houses and pubs. They are generally performed seasonally, often at Christmas, Easter or on Plough Monday, more rarely on Halloween or All Souls' Day, and often with a collection of money. The practice may be compared with other customs such as those of Halloween, Bonfire Night, wassailing, pace egging and first-footing at new year.

Although the term mummer has been in use since the Middle Ages, no scripts or details survive from that era and the term may have been used loosely to describe performers of several different kinds. The earliest evidence of mummers' plays as they are known today is from the mid- to late 18th century. Mummers' plays should not be confused with the earlier mystery plays.

Mumming spread from the British Isles to a number of former British colonies. Ireland has its own unique history of mummers' play, and adopted the term for the tradition from the English language.

The word mummer is sometimes explained to derive from Middle English mum ("silent") or Greek mommo ("mask"), but is more likely to be associated with Early New High German mummer ("disguised person", attested in Johann Fischart) and vermummen ("to wrap up, to disguise, to mask one's face"), which itself is derived from or came to be associated with mummen (first attested already in Middle High German by a prohibition in Mühlhausen, Thuringia, 1351) and mum(en)schanz, (Hans Sachs, Nuremberg, 16th century), these latter words originally referring to a game or throw (schanz) of dice. Ingrid Brainard argues that the English word "mummer" is ultimately derived from the Greek name Momus, a god of mockery and scoff.

Mummers' and guisers' plays were formerly performed throughout much of English-speaking Great Britain and Ireland, spreading to other English-speaking parts of the world including Newfoundland and Saint Kitts and Nevis. There are a few surviving traditional teams of mummers in England and Ireland, but there have been many revivals of mumming, often associated nowadays with morris and sword dance groups. These performances are comparable in some respects with others throughout Europe.

Broadly comic performances, the most common type features a doctor who has a magic potion able to resuscitate the vanquished character. Early scholars of folk drama, influenced by James Frazer's The Golden Bough, tended to view these plays as descendants of pre-Christian fertility ritual, but modern researchers have subjected this interpretation to criticism.

The characters may be introduced in a series of short speeches (usually in rhyming couplets) or they may introduce themselves in the course of the play's action. The principal characters, presented in a wide variety of manners, are a hero, most commonly Saint George, King George, or Prince George (but Robin Hood in the Cotswolds and Galoshin in Scotland), and his chief opponent (known as the Turkish Knight in southern England, but named Slasher elsewhere), and a quack Doctor who comes to restore the dead man to life. Other characters include: Old Father Christmas, who introduces some plays, the Fool and Beelzebub or Little Devil Doubt (who demands money from the audience).