Separated at Birth

A plot in which two siblings (usually identical twins) who were separated at birth now meet up as youngsters or adults. Usually the twins are mistaken for one another by family/friends and Hilarity Ensues (e.g. The Parent Trap). Often the two characters were previously unhappy with their lots in life, but after having walked a mile in somebody else's shoes, they gain a new appreciation for what they have. Notice that Separated at Birth doesn't have to apply literally - their parents might have divorced, and each parent took one kid and neither kid was told of the other's existence, but before that they were one family for some time. But Separated At Birth, especially if combined with Death By Childbirth, can make for so much drama.

A plot which sometimes rises from this is Twincest. This is also a good way to drop an Evil Twin on your doorstep, though that variant is a Dead Horse Trope by now.

A Speculative Fiction variant is Split At Birth. Sub-trope of Long-Lost Relative. Compare Switched At Birth, where one baby is swapped for another. For twins literally separated at birth, see Conjoined Twins.

Anime

 * Gundam Seed, due to the fact that Kira and Cagalli, despite being twins separated at birth and raised by different parents. It should also be noted that other materials state that Kira's adoptive Hot Shounen Mom Caridad Yamato is, in fact, his and Cagalli's maternal aunt (the sister of Via Hibiki). This is a Homage to Star Wars anyway...
 * Due to the Only Six Faces rule of this show, many viewer didn't realize that Kira and Cagalli are twins until it was pointed outright. And then there's numerous Ship Tease...
 * Part of a subplot in Digimon Frontier, where two boys named Kouichi and Kouji discover they are twin brothers, and one of them is lured to the dark side. It takes a lot for his brother to make him pull a Heel Face Turn.
 * Heavily subverted in Gundam 00.
 * In the Saiyuki manga, Hakkai and his twin sister were placed in different orphanages shortly after birth, but met and fell in love. Even after learning their identities as brother and sister, they continued the affair.
 * Momiji and Kaede of Blue Seed are an unusual example of this trope in that they are fraternal twins. No one would mistake one for the other but the fact that they are twins (and Kaede is the eldest, if only by minutes) has serious supernatural consequences over the course of the story since it dilutes their hereditary magical powers.
 * Goku and Raditz in Dragonball Z. It's a non-twin case. It's apparantly common for Saiyans to send babies to planets with weak people to kill off the population.
 * Hiei and Yukina of Yu Yu Hakusho are this. Well, it varies, depending on the translation, or if you believe what Hiei has to say in the dubs. The generally accepted belief is that they're twins separated at birth; Hiei was cast off the Koorime Island for being born male in their female-only race, while Yukina was allowed to stay with their mother. While they do meet in the series and are often in each other's company, Hiei is the only one of the two definitely aware of their relationship while it was never made clear if Yukina was aware herself.
 * Popular fan theory, along with other variants of Long-Lost Relative, for Detective Conan's lead Kudo Shinichi and his Foil and Worthy Opponent the Kaitou Kid. (Who had his own very brief manga series before Aoyama started Conan and folded Kid into it.) Interesting meta-use of Only Six Faces, since what the manga-ka actually did was use the same basic design for two heroes and then Lampshade it--specifically, by showing that Kid, a Master of Disguise who usually relies heavily on Latex Perfection, can dress as Shinichi without a mask. Just has to change his hair and body language.
 * The twin explanations are actually more complicated than the cousin etc. examples, since both of their fathers appear, though Kid's only in flashback and a portrait--and also look pretty much identical, except for their styles of moustache. Their mothers are very different.
 * A fun one had them as children of a similar-looking mutual cousin of both Touichi and Yuusaku, who suffered Death By Childbirth while being pursued by the evil syndicate.
 * In the Code Geass spinoff manga Nightmare of Nunnally,  are examples.
 * Though they could both fit the criteria for Evil Twin,  who defiantly wins the part of the Even More Evil Twin.
 * from the anime Another.

Comic Books

 * Northstar and Aurora of Alpha Flight were separated as infants when their parents died. They quickly developed a very... intense sibling relationship, despite not having been raised together.
 * Batman villains the Trigger Twins think this is what happened to them. They don't know for sure, but there aren't a lot of other explanations for two complete strangers who look exactly identical to one another. One of them even mentions something he saw on TV about separated twins leading unusually like lives as adults... and the Trigger Twins met when they tried to rob the same bank.
 * The Mark Waid run of The Flash introduced Malcolm Thawne, Barry Allen's identical twin who was Switched At Birth. However, while an earlier encounter was retconned in, their biological relationship didn't come out until after Barry was dead.

Film

 * Austin and his are separated in Austin Powers, with one being raised by a negligent spy father and the other raised by.
 * Luke and Leia in Star Wars were Separated At Birth to hide them from their father.
 * This trope was subverted in the movie Twins with Schwarzenegger and Devito as the twins, who cannot be mistaken for each other. The Olsens also did a sort of subversion of this in It Takes Two, where (for once) they're not playing sisters.
 * The Jackie Chan film Twin Dragons.
 * The Jean Claude Van Damme film Double Impact.
 * Jean Claude Van Damme loves this trope, he did it again in Maximum Risk where one of the twins dies before the start of the movie but not before finding out all about his long lost brother, and forcing the brother to pose as him to uncover who killed him. And again in the Replicant but here the twin is actually a clone. Still the plot of the movie pretty much sets him up as a separated at birth, good, twin to the originals evil twin (complete with Twin Telepathy).
 * The entire plot of Big Business -- times TWO (it's a 1980s update of The Comedy of Errors).
 * Bette Midler and Lili Tomlin star as two sets of twins: the Midlers come from rich city parents, the Tomlins come from poor country parents. Both families end up with one Midler (Sadie) and one Tomlin (Rose) each. The movie is one part Switched At Birth, one part Separated At Birth, one part The City Mouse And The Country Mouse. Hilarity ensues.
 * One word: Jesus.
 * Start the Revolution Without Me has a similar situation as Big Business: two sets of identical twins (one noble, and one peasant) are born at the same time and placed on the same bed. Since nobody could figure out which set of twins belonged to which set of parents they decided to re-combine the pairs since that guaranteed they'd be "half right".
 * The basic premise of The Parent Trap, in all its various remakes.
 * My Twentieth Century
 * Raymond and Charlie from Rain Man
 * Likewas not twins. Raymond was older and he was institutionalized when Charlie was a baby.
 * In Tom & Thomas,

Literature

 * In Billy and Howard, one of the title characters is left to live among normals for a decade, to give perspective to his controlling twin when they are reunited. He is able to confront his creator in the second book about this. He is not pleased.
 * The German novel Das doppelte Lottchen (translated as Lottie and Lisa) by Erich Kästner, and The Film of the Book The Parent Trap.
 * Amoret and Belphoebe in The Faerie Queene
 * Twin sisters Cam and Alex from Twitches were separated at birth and adopted by different families. They met each other when they were 14, and found out they were witches.
 * Subverted in The Big Four by Agatha Christie, where
 * Lisa Scottoline's Bennie and Alice in Mistaken Identity, Dead Ringer and Think Twice.
 * A particularly freakish example in Acorna's Children: Second Wave, in which we learn that.
 * An example of this occurs with Non-Identical Twins in the In Death series. The two boys met in the army, went through everything together: spec ops, evil paramilitary organizations, serial killing... you know, brother stuff.
 * Sam and Caine in the Gone series. Take a long look at Caine's name, and you have the nature of their relationship.
 * The Lois Duncan book Stranger with my Face is based entirely on this trope. The main character was adopted at birth, but her parents decided not to take her twin sister because they could sense her evil. The other twin grows up to come and ruin the heroine's life, using, of all things, astral projection. This becomes rather perplexing when the main character and her family are described. They're all blond, pale, and white, while she's half Native American.
 * Cor (Shasta) and Corrin from The Horse and His Boy in the Chronicles of Narnia.
 * in The Alleluia Files.

Live Action TV

 * Two shows that get significant mileage on this trope are Sister Sister (with the Mowrys) and So Little Time (with the Olsens).
 * The series Two, where the main characters evil twin, separated at birth and non-existent in official documents, sets out to destroy the main character's life, at first killing his family, making him flee the authorities. A common element in this show was that the bad guy deliberately got caught on tape to frame his brother. Of course, neither of them appear at once in the same scene.
 * This trope served as a plot on the soap opera One Life to Live in the mid-90s. Maggie Carpenter and her long-lost twin brother Ian were separated by their parents, who were each married to someone else; the twins had been the result of their adulterous affair. The father raised the daughter and the mother raised the son. The twins met as adults, but fortunately did not develop an attraction to each other.
 * A tragic example is the episode "Happenstance" on CSI, where the victims were identical twins murdered on the same night and they had no knowledge of each other's existence.
 * In the re-imagined Flash Gordon series, are twins, but  was supposed to have been killed for being a Deviant (a mutant). They almost have a Twincest moment, but Flash manages to stop them, revealing the truth. Later, it is revealed that the Deviant DNA came from the father, a Deviant-hater.

Theatre

 * This trope forms the basis for Shakespeare's The Comedy of Errors, which was itself cribbed from Plautus's Menaechmi, making it Older Than Feudalism.
 * It's also the plot of Willy Russell's Blood Brothers. The boys meet as children and become friends into adulthood... it doesn't end well.
 * Oscar Wilde's The Importance of Being Earnest deals with a man who was lost by his governess a few weeks after birth. He eventually meets up with his younger brother while pretending to be a fictional version of said brother, and they both end up getting married. Oh, and the name 'Earnest' comes into play more than once, used by both brothers....

Video Games

 * In Ever 17 fraternal twins were anonymously placed in an orphanage by their mother, who was on the run from an Evil Corporation and hoped to keep them safe. Unfortunately the corporation managed to track them down and kidnap them both, ending up separating the two twins to really put the boot in.
 * In the PopCap Games title AstroPop, turn out to be fraternal twins.

Web Comic

 * Elan of Order of the Stick and his Evil Twin, Nale.
 * Walky and Sal, in It's Walky!, though it's hard to see why they didn't figure it out sooner. In this case, Sal was switched with another child (Beef) to generate the impression she was more important and needed hiding when the more important one was actually Walky. Sal herself, and for a long time the reader, are fooled too. Tragically, their reunion ultimately triggers Beef's Face Heel Turn.
 * The Inexplicable Adventures of Bob: Although artificial creature Molly and her identical "sister Golly were born in different places a few months apart, most aspects of this trope still apply. They had radically different upbringings giving them very different personalities, they only met after they were both fully-grown, Golly was initially mistaken for Molly, and each regards the other as an object lesson in what they might have become under other circumstances (the story title is "There But For the Grace," as in, "There but for the grace of God go I."'')
 * Homestuck:  God bless Time Travel.

Western Animation

 * This trope was given a passing mention on The Simpsons, where "Bleeding Gums" Murphy, when asked about his family, mentions a little brother who became a doctor and "used to laugh at the most inappropriate times". Dr. Hibbert, meanwhile, makes reference to an older brother who's "a jazz musician or some such".
 * The director of the Shelbyville orphanage also mentions he has a long lost brother - he is identical in looks to Dr Hibbert, but Homer is too interested in his own long lost brother Herb to point that out.
 * In neither case are the siblings implied to be twins, however.
 * There was a Treehouse of Horror segment where Bart had an identical twin named Hugo who lived in the attic. They were separated because one of them was evil. Bart was revealed to be the evil one and switched places with Hugo.
 * Bart and Hugo were also Siamese conjoined twins (separated by Doctor Hibbert).
 * Prince Adam and Princess Adora, the secret identities of He-Man and She-Ra in He Man and The Masters of The Universe cartoon and its Spin-Off She Ra Princess of Power, were separated at birth, unintentionally. Hordak, the Big Bad of the She-Ra series, intended to abduct both of the infant twins but had only managed to pick up Adora before he was interrupted by the palace guards, taking the little girl away and raising her as a Tyke Bomb. Sorceress had to give everyone in the kingdom (except the royal family, of course) Laser-Guided Amnesia to cover the incident up, and Adam/He-Man himself had to help Adora/She-Ra recover her identity in the She-Ra pilot.
 * The cartoon version of Double Dragon pulled this with Billy and Jimmy Lee, with Billy being raised by the benevolent "Eldest Dragon" to become the Dragon Master, while Jimmy was raised by the evil Shadow Master to become his second-in-command, the Shadow Boss. Eventually Jimmy does a Heel Face Turn when the Shadow Master betrays him and teams up with his brother for the rest of the series.

Real Life

 * Twins which are separated are often sought out for statistics based studies, as they have the advantage of having identical DNA but being raised in different environments (which makes it easier to study the genetic factors involved in various illnesses that they may have). These studies tend to be limited in scope and produce unreliable data, however, because of the small sample size and the fact that the twins would still have shared the same environment as they developed in the womb.
 * Happened in England in 2008. The twins had been adopted separately, met up, fell in love... Got married, found out because of this, and separated.
 * In fiction, Obi Wan Kenobi intervenes before it's too late.
 * It's never too late for Rule 34.
 * Happened in Poland. One of twins was accidentally exchanged for another girl in hospital. It came out no sooner than about twenty years later, after a friend of one twin took the second for her. The whole trio has severe identity problems because of this.
 * On Oprah, there were 6 year old twins from China who had been separated in the orphanage and given to different parents.
 * Both twins were unknowingly given the same name (Mia).
 * This seems to be more common in China than in other countries; a recent National Geographic expose on identical twins told the story of two couples who sought to adopt children from China. The couples went together, and discovered that the children they wanted to adopt were twins. For some odd reason, the orphanage would not allow the girls to be adopted into the same family; the couples were told that either the girls were separated by them or they would find two other couples to do it. The couples feared that if the girls were adopted by others, they could be separated for life. So, each couple adopted one of the girls and since they live rather close to each other, the girls visit regularly.