It's a funny custom, isn't it? Dressing up like ghouls and goblins, and going from door-to-door to beg for fistfuls of usually forbidden treats... whose idea was that? 

The custom of trick-or-treating may have Celtic origins, related to the pagan celebration of Samhain, which marked the end of the harvest and the threshold of a new season. According to anthropologist Bettina Arnold:


The association between Halloween and ghosts and spirits today comes from the Celtic belief that it was at this time of transition between the old year and the new that the barrier between this world and the Otherworld where the dead and supernatural beings lived became permeable....Trick-or-treating is a modern day holdover of the practice of propitiating, or bribing, the spirits and their human counterparts roaming the world of the living on that night. Pumpkins carved as jack-o-lanterns would not have been part of traditional Halloween festivals in Celtic Europe, since pumpkins are New World plants, but large turnips were hollowed out, carved with faces and placed in windows to ward off evil spirits.

Others argue that Halloween is a Christian, not a pagan holiday, pointing to the early Catholic church's celebrations of All Hallows (Saints) Day, and the night before it, All Hallows E'en (Evening), when Christians were instructed to pray for the souls of the departed. I can see how that would lead to a certain fascination with ghosts, but the candy? Well, back in medieval Europe, kids and beggars would go "souling" on All Hallows Eve...which sounds like a macabre version of door-to-door Christmas caroling: Instead of a merry song, the visitors offered prayers for dead loved ones, in exchange for "soul cakes." (These, too, may have had pagan roots.) 

Some chap named Charles Dickens mentions this tradition in an 1887 issue of his literary journal, " All the Year Round" (actually, I think it must have been Charles Dickens, Jr., who took over the journal after his dad died in 1870):

"...it was a custom to bake on All Hallow E'en, a cake for every soul in the house, which cakes were eaten on All Souls' Day. The poor people used to go round begging for some cakes or anything to make merry with on this night. Their petition consisted in singing a doggerel sort of rhyme: A soul cake, A soul cake; Have mercy on all Christian souls; For a soul cake; A soul cake. In Cheshire on this night they once had a custom called 'Hob Nob,' which consisted of a man carrying a dead horse's head covered with a sheet to frighten people."

Eep! That's quite a trick, alright. In America these days, not too many people take the "trick" part of trick-or-treating seriously anymore; it's more like: "Hi, gimme candy." But according to this New York Times article, Halloween night trickery is a problem in the United Kingdom, where "egg-and-flour-throwing, attacks on fences and doors and the theft of garden ornaments" are enough to make some people— gasp!—"hate Halloween."