Once Upon a Time is an American fantasy adventure drama series that aired for seven seasons on ABC from October 23, 2011 to May 18, 2018. The action alternates between two main settings: a fantastical world where fairy tales happen and a fictional seaside town in Maine called Storybrooke. The "real world" part of the story unfolds with the characters of Emma Swan (Jennifer Morrison) and her 10-year-old son, Henry Mills (Jared S. Gilmore). Henry discovers the other people of the town are fairy-tale characters. The audience is shown the backstory of the town's people as fairy-tale characters, in conjunction with their unfolding stories in the "real-world". In the seventh and final season, the "real-world" portion of the story takes place in Seattle, Washington, in the fictitious neighborhood of "Hyperion Heights", with a new main narrative led by adult Henry (Andrew J. West), and his wife and daughter.

Once Upon a Time was created by Lost and Tron: Legacy writers Edward Kitsis and Adam Horowitz. Horowitz told The Hollywood Reporter that they had a conscious purpose to "do a show that had optimism at its heart", as they "felt like there was so much darkness in the world". Kitsis said: "We are guys who like to see the world as half-full, not half-empty", and it's about "seeing that among all the trials and tribulations of life, you can persevere and find light among the darkness"; "it's that ability to think your life will get better". Star Jennifer Morrison told the Calgary Herald that it's a show about hope and positivity and connectivity, which "encourages people to believe in themselves and believe in the best versions of themselves and to have hope to have the life that they have."

A spin-off series, Once Upon a Time in Wonderland, consisting of 13 episodes featuring the title character of the 1865 novel Alice's Adventures in Wonderland by Lewis Carroll, premiered on October 10, 2013, and concluded on April 3, 2014.