"Once upon a time" is a stock phrase used to introduce a narrative of past events, typically in fairy tales and folk tales. It has been used in some form since at least 1380 (according to the Oxford English Dictionary) in storytelling in the English language and has started many narratives since 1600. These stories sometimes end with "and they all lived happily ever after", or, originally, "happily until their deaths".

The phrase is common in fairy tales for younger children. It was used in the original translations of the stories of Charles Perrault as a translation for the French "il était une fois", of Hans Christian Andersen as a translation for the Danish "der var engang" (literally "there was once"), the Brothers Grimm as a translation for the German "es war einmal" (literally "it was once") and Joseph Jacobs in English translations and fairy tales.

In More English Fairy Tales, Joseph Jacobs notes that:

"The opening formula are varied enough, but none of them has much play of fancy. 'Once upon a time and a very good time it was, though it wasn't in my time nor in your time nor in any one else's time.' is effective enough for a fairy epoch, and is common, according to Mayhew (London Labour, III), among tramps."^[1]

The phrase is also used in retellings of myths, fables and folklore.^[2]

The "story-starting phrase" is a common feature of many languages.

• Don McLean's "American Pie" begins with the phrase "A long, long time ago...".
• All of the Star Wars films, as well as several of the expanded universe novels, begin with the phrase "A long time ago in a galaxy far, far away....".
• The musical Into the Woods begins with the Narrator's line, "Once upon a time." The second act commences with his line, "Once upon a time... later..." The musical is a retelling of many famous fairy tales.
• In the Singaporean comedy series Under One Roof, Moses Lim's character Tan Ah Teck begins his stories with "Long before your time, in the southern province of China ...".
• In the 2010 Edgar Wright film Scott Pilgrim vs. the World, the opening scene narrates the words "Not so long ago, in the mysterious land of Toronto, Canada."
• The opening line of the theme song to MST3k is: "In the not-too-distant future ... next Sunday, A.D."
• Bionicle features the line "In the time before time...".
• The opening line of the TV series Merlin features the line "In a land of myth, and a time of magic..."
• The first episode of Ivor the Engine opens with the line "Not so very long ago, in the top left-hand corner of Wales..."

• In the beginning (phrase)
• All's Well That Ends Well
• Happy ending
• It was a dark and stormy night