Most episodes of Lost start with a recap sequence, with the words "Previously on Lost" heard at the beginning. The voice is that of former ABC chairman Lloyd Braun, who originally pitched and commissioned the Lost pilot. These words are followed by a series of short clips that recap the scenes that are important for the current episode. The purpose of these "Previously on Lost" clips is to both remind viewers about current and past storylines, as well as aid comprehension for those who may not have seen the last few episodes of the show. These segments last for approximately one minute, forty-five seconds, and include original music by composer Michael Giacchino.
In some cases (such as in "Orientation" and "There's No Place Like Home, Part 2"), the episode proper begins with a scene that is a direct continuation of a previous scene, and thus the "Previously on Lost" segment figures into the narrative flow of the story. Sometimes, speech and actions are changed or edited from their original versions. Originally, in "Live Together, Die Alone", Benjamin Linus said, "Your friends are coming home with us." In the "Previously on Lost" for "A Tale of Two Cities", Ben says, "You're coming with us." The "Previously on Lost" from "Because You Left" features a previously unseen overhead shot of the US Coast Guard plane as the Oceanic Six are greeted by their family members.
The "Previously on Lost" from the episode "Orientation" sees the following concept:
- "Previously on Lost" is said, during which a scene is shown of Jack and Desmond running at the stadium. There is a condensed version of events, including Desmond saying that he is training for a race. The two also exchange names.
- On the Island, Jack investigates the Swan and notices the magnetic force acting on the key around his neck.
- Kate is shown tied up, trying to get out, while Locke is being ordered to type in the Numbers.
- The dramatic cliffhanger is shown between Jack, Desmond and Locke. [End dramatic music]
- Michael and Sawyer arrive on land after the destruction of the raft, and Jin comes out of the jungle followed by the tail section survivors, who they think are Others. [Smash out]
Episodes without a "Previously on Lost"
|Season 1||Season 2||Season 3||Season 4||Season 5||Season 6|
|"Pilot, Part 1"
|"The Other 48 Days"
||"Tricia Tanaka Is Dead"
||"LA X, Part 2" (available if episode is shown separate from LA X, Part 1)|
The voice of "Previously on Lost" is that of former ABC chairman Lloyd Braun. J.J. Abrams asked Braun to do the voiceover as an homage for his role in the making of the show, after Braun was fired from ABC following his commissioning of the Lost pilot. Braun agreed on the condition of anonymity. Abrams, Braun, and a sound man met in a conference room at the Beverly Hills Hotel and recorded the line. According to Braun, the recording was lowered in pitch somewhat to help disguise the voice. The identity of the voice was kept secret for a few years. According to Braun, it was Howard Stern, a close friend of his, who found out and leaked the identity. 
Before it was openly disclosed that Braun was the voice, many rumors and guesses as to the voice were alleged, and producers upheld the mystery of the identity:
- In the February 13, 2006 Official Podcast, Carlton Cuse said the "Previously on Lost" voice is a "mystery person". Cuse also denied being the voice.
- As Damon Lindelof read him the question, he instructed Carlton not to reveal the true identity of the person.
- They also specifically denied that Tim Daly was the voice.
- In a July 14, 2006 post on thefuselage.com, script coordinator Gregg Nations said that the voice was of Carlton Cuse. 
- During the DVD commentary for "There's No Place Like Home, Part 2", Damon Lindelof mentioned that, contrary to popular belief, the voice is not his.