Trick-or-treating is a customary celebration for children on Halloween. Children go in costume from house to house, asking for treats such as candy or sometimes money, with the question, "Trick or treat?" The word "trick" refers to a (mostly idle) "threat" to perform mischief on the homeowners or their property if no treat is given. In some parts of Scotland children still go guising. In this custom the child performs some sort of trick, i.e. sings a song or tells a ghost story, to earn their treats.

The practice of dressing up in costumes and begging door to door for treats on holidays dates back to the Middle Ages and includes Christmas wassailing. Trick-or-treating resembles the late medieval practice of souling, when poor folk would go door to door on Hallowmas (November 1), receiving food in return for prayers for the dead on All Souls' Day (November 2). It originated in Ireland and Britain,^[14] although similar practices for the souls of the dead were found as far south as Italy.^[15] Shakespeare mentions the practice in his comedy The Two Gentlemen of Verona (1593), when Speed accuses his master of "puling [whimpering or whining] like a beggar at Hallowmas."^[16]

In Scotland and Ireland, Guising — children disguised in costume going from door to door for food or coins — is a traditional Halloween custom, and is recorded in Scotland at Halloween in 1895 where masqueraders in disguise carrying lanterns made out of scooped out turnips, visit homes to be rewarded with cakes, fruit and money.^[8] The practise of Guising at Halloween in North America is first recorded in 1911, where a newspaper in Kingston, Ontario reported children going "guising" around the neighborhood.^[17]