Alien Among Us

"I'm just a perfectly normal human worm baby! You have nothing, absolutely nothing, to worry about."

- Zim the Alien, Invader Zim

A being from outer space generally comes to Earth either to observe and learn about us or to help guide us (or just get stuck here) and must keep his true nature a secret. A good opportunity for Mr. Exposition to make his presence felt as the alien often needs everything explained, or describes the situation for others of his kind who might share his curiosity. A lot of comedy can be drawn from the newly landed alien having No Social Skills.

Alternatively, the alien or aliens could be stranded and unable to leave. If they want to return home, they'll never get to, making Earth their Gilligan's Island. Often, the alien will eventually lose interest in leaving, and if given the opportunity will refuse it. This is usually the case when they form strong attachments and relationships to Earthlings, such as becoming friends with a human roommate.

Tends to grow less and less plausible the longer a series lasts, as evidenced by the difficulty many viewers had with the idea that the 3rd Rock from the Sun aliens would not know about taxes after four years in the United States.

If the main character is from our world (or one close to it) and the setting is another, that's Trapped in Another World.

See also Amusing Alien, Humanity Ensues, Raised by Wolves, Fish Out of Water, Mysterious Waif.

Anime and Manga

 * Haruhi Suzumiya: Yuki has the major stuff down, but when you get down to the details like how a computer is used or maybe what a glasses fetish happens to be... Also Kuyou, the resident Starfish Alien, who is so different that she can't really have the concepts explained to her.
 * This was the starting premise of Kenichi Sonoda's Cannon God Exaxxion manga, where aliens called the Riofaldians came to earth in peace, bringing their advanced technology with them and have been coexisting with humans on earth for several years. Of course, they were secretly infiltrating us to set us up for conquest all along (the weapons technology they gave us is ineffective against their REAL weapons) and this appears to be standard operating procedure for them.
 * Dragon Ball: Pretty much every other character, mainly Goku, Piccolo and Vegeta.

Comic Books

 * In some incarnations, J'onn J'onzz, the Martian Manhunter, fits this trope.
 * So does the Marvel hero Sleepwalker, who had his own comic book in the early 1990s. Fortunately, since he spent all his time fighting monsters in his home dimension, he was able to make the best of being trapped on Earth by fighting supervillains and catching criminals, all while defending humanity from demons native to the Mindscape.
 * Icon. When his ship crashed on Earth, he automatically assumed a human shape, but as he was found by a plantation slave in 1839 and has extreme longevity, his experience is markedly different for a Superman Expy. He becomes The Cape (trope) when young Raquel Ervin urged him to use his powers to be an inspiration, making herself the sidekick Rocket in the process, but doesn't give up his hope of returning to his home world.

Film

 * Station engineer Philo in UHF.
 * The Men in Black series.
 * It Came from Outer Space (1953) has the alien cat Jake crash-landing on Earth. Aware that his presence may cause conflict, so he tries to leave as quickly and quietly as possible.
 * The Witch Mountain series.
 * Taken to its least idealistic extreme in District 9, where the sick, vulnerable aliens wind up stranded in Johannesburg.
 * Starman
 * Beldar and his wife ended up stranded on Earth in Coneheads.
 * Cowboys and Aliens has.

Literature

 * In the book and film The Man Who Fell to Earth, the being that calls itself Thomas Jerome Newton comes to Earth in order to help his own people, and in doing so embetters humanity as well by introducing alien technologies to Earth. In the end,.
 * Zenna Henderson's The People stories are about a race of Human Aliens with Psychic Powers who fled to Earth after their homeworld was destroyed.
 * Ford Prefect in The Hitchhiker's Guide to the Galaxy, in the fifteen-year exile set before the main action of the first book.
 * Although it does carry on through approximately half an hour of the beginning.
 * In the short story "On a Clear Day You Can See All the Way to Conspiracy" by Desmond Warzel, there are two aliens among us—one was sentenced to live here, and the second is here to keep an eye on the first guy.
 * Subverted strongly in the Lawrence Watt-Evans short story One Of The Boys, a Deconstruction of Superman, about an alien that looks human and was raised on Earth all his life, but is still painfully, dangerously alien.
 * The children's book "Stinker from Space" by Pamela F. Service is about an alien who is fatally wounded when he crash lands on Earth while fleeing enemy aliens. He has the ability to transfer his consciousness/intelligence into other bodies, which will save his life. Unfortunately, the only compatible body within range is that of a skunk. Hence the title. He spends much of the book trying to pass himself off as a tame, deodorized skunk so that people won't shoot him while he tries to repair his ship in secret.
 * Ax in Animorphs. He actually spends most of his free time hiding in the woods, but occasionally morphs into a human form and tries to pass as an ordinary human. Given that his mouthless species doesn't speak verbally or have a sense of taste, it takes him a long time to get the hang of it.
 * My Summer On Earth
 * Stepsister From The Planet Weird
 * "I'm A Stranger Here Myself", a short story by Mack Reynolds. Two western expatriates in Tangier are discussing a news article on the Flying Saucer craze, and the usual Alien Among Us theories. One scoffs at the idea, pointing out that with the vast number of police, security and counter-intelligence agents on Earth, one of these alien observers would be bound to slip up and get caught. His companion responds that Tangiers, as opposed to one of the major capitals like New York or London, would be a perfect place to hide out, as no-one plays attention to anyones' business. It turns out that both men are aliens, but rather than being there for the expected noble reasons, one is harvesting human protein, the other is stirring up wars and tribal conflicts for alien thrill tourists. Which, the first one points out sourly, could spoil an awful lot of good meat.
 * A large part of the Lorien Legacies series, and the I Am Number Four movie.
 * Mikhail Akhmanov's Arrivals From the Dark series has an alien character whose name roughly translates as Exile. He is a Proteid (or Metamorph), a species of Voluntary Shapeshifters, whose hat is observing other races using their shapeshifting and Psychic Powers. Exile has a rare genetic disorder that only allows him to fully shapeshift once, at which point he is stuck in this form for life (and they're, effectively, immortal), only able to make small cosmetic alterations. He chooses to be an observer on Earth and arrives in the 13th century during the Mongol invasion. He has secretly observed humanity, only interfering occasionally to subtly boost progress. When the Faata arrive to conquer Earth, humanity is far from ready to take them on, still being stuck in the Solar System and lacking advanced weapons. Exile gives the humans the means to destroy the Faata ship in such a way that it can be studied and its technology reverse-engineered. Since then, Exile has taken a slightly more active role in helping humanity, although he has always remained in the shadows, revealing himself to a select few individuals and changing identities (and faces) every so often.
 * Inverted in the Trevelyan's Mission books, which take place centuries later in the same 'verse, with the titular protagonist and his colleagues, whose job is to infiltrate primitive humanoid cultures in order to study them and accelerate their progress.

Live Action TV

 * My Favorite Martian
 * 3rd Rock from the Sun
 * Roswell
 * Out of This World
 * Only insomuch as Evie occasionally explained Earth concepts to her alien dad. But this trope was not a major feature of the show, since Evie was raised on Earth and completely familiar with its culture.
 * My Hero (TV)
 * Tracker
 * Cole/Dagon doesn't really make much effort to blend into human society. His speech is strange and monotonous, although it gets better in later episodes. He has trouble understanding things we take for granted (such as trying to explain to Mel, who knows who he really is, what Star Wars is). Even bathing is new to him. I guess they don't do that on Cirron.
 * ALF crash landed, and had little motivation to leave.
 * ... but had little problem adapting.
 * Hey, they never once mentioned him actually eating a cat while on Earth, something he did regularly on Melmac.
 * The Tenctonese/Newcomers on Alien Nation were refugees and slaves. In this case it was over 100,000 aliens, who form their own community in Los Angeles. Therefore the cultural misunderstandings went both ways, as humans learned to deal with odd newcomer traditions.
 * The character of Anya from Buffy the Vampire Slayer could also be considered a variation on this device. Anya was a 1,120-year-old former demon, who did not understand the conventions of humans and, therefore, had wacky misunderstandings and needed to have things explained to her. Anya eventually subverted this trope in a (according to some) Good Troi Episode which flashed back to show her in her pre-demonic days. Turns out that her inability to grasp the conventions of humans has less to do with her being a former demon, and more to do with her just being very literal-minded.
 * Mork and Mindy once had a clever Lampshade Hanging about this trope when, in one of the last seasons of the show, Mindy exclaimed in exasperation, "Oh, Mork, what Earth concept have you misunderstood this week?"
 * The TV series Hard Time On Planet Earth starred Martin Kove as an alien criminal sentenced to serve out his time on the unimportant backwater planet Earth. Accompanied by his "warden" (a computer-animated flying eye), the protagonist used his inhuman strength to fight bad guys while Walking the Earth
 * The short-lived Nickelodeon show The Journey of Allen Strange, about a young Energy Being living incognito as a human on Earth.
 * My Parents Are Aliens in which the aliens adopt some kids as part of their cover story.
 * My Best Friend Is An Alien is a rare version where the Rubber Forehead Aliens are living publicly among humans, and the main character is the son of a diplomat at an alien embassy.
 * The Powers of Matthew Star combined learning about Earth with How Do I Shot Web?.
 * Bibi from the French Canadian kid's show "Bibi et Genevi?", later translated as BB & Jennifer was an alien from the planet XY1000Z come to earth to study it.
 * On Top Gear, one possible explanation for The Stig. Granted, there are many...
 * The entire fourth season of Lexx uses this to hilarious effect. The crew of the titular ship are from a society so intrinsically different than any on Earth that even when they do figure out how something works, they're completely flabbergasted about it. Examples include confusion over how much machinery and power is used by the space shuttle just to reach the moon, as well as being unable to comprehend the practice of burying the dead; since the dead were processed for protein back on the Cluster, this seems like, literally, a waste of resources.
 * The core plot of the short-lived sitcom Meego.
 * On the short-lived Jim Henson Productions sitcom Aliens in the Family, an alien and a human fall in love and form a blended family. Oddly enough, their family (which includes a Muppet baby and two older aliens in full body costumes) lives on Earth.
 * Trip from Power Rangers Time Force, sort of, since apparently aliens aren't as common in the year 2001 as they are in the year 3000.
 * The Doctor generally avoids telling everyone that he's an alien time-traveler, although certain fortunate people do get to find out. He even has a psychic paper to help him avoid rousing suspicions when he investigates.
 * Ultra Seven

Tabletop RPG

 * In the Alternity / D20 Modern setting Dark•Matter there are several aliens who hid on Earth, with some who tried to prepare them for the High Tide.

Toys

 * The premise of most Transformers series starts here.

Video Games

 * In Super Robot Wars 4, Super Robot Wars Compact and Super Robot Wars GC, Heavy Metal L-Gaim shows up in the game and its protagonists observe the other characters and the overarching plot.
 * Escape From St Marys: There's an alien in the school, says an agent you meet. Whoever it is has decided to blend as a student or teacher.

Web Comics

 * In Sluggy Freelance Aylee is a double duty alien (she's from another planet and from another dimension). Even after 11 years, she's still very much the Raised by Wolves character.
 * Though, to be fair, she spent a number of those years stuck inside her cocoon.
 * And now that she's metamorphosed to 'almost human form, clothes and all', it's close to becoming a case of Humanity Ensues, except for the persistent Raised by Wolves thing. And the teeth.
 * Killroy of Killroy And Tina is a warlord who was sent to Earth and bonded with the other title character both as punishment and as Gambit Roulette.
 * Summer's classmate Uma from Everyday Heroes. Even though she and her father resemble cows, no one has mentioned this or even noticed ... apparently it's Someone Else's Problem.
 * Done to an extent in Medieval, where the characters are aliens but also bear a distinct likeness to twenty-first century humans in terms of ideas and culture. Unfortunately for them, they landed in Earth's 12th century Europe.
 * El Goonish Shive has the Uryuom, an entire species of Aliens living on Earth more or less unnoticed by all but a few. Some even object to being characterized as aliens since they were born here.
 * The question is, how long? In an Alternate Universe they were living among the humans openly at least from the time of American Revolutionary War. When they have arrived is unknown, but the Uryuom team on their original quest ran into the local acting god of magicspoilers, which resulted in a Curb Stomp Battle, presumably followed by a change in rules of magic, as he warned them (we know Uryuom shapeshifting became mostly incompatible with human magic, which hinders acquisition of other magic abilities). But these events are rare: the last change before "current" era happened during Pandora's previous life, at very least five centuries ago, probably more — and apparently in response to actions of humans, thus most likely wasn't that one.
 * Girls in Space main character Zoe Strider is marooned on Earth, in Edinburgh, Scotland
 * In Jix the titular character, Caligos, Dyonus, and Dyona (latter two being droids made by the aliens) are all aliens living with a Polynesian couple.

Western Animation

 * American Dad's Roger is an alien that freeloads off the Smith family, in a seeming parody of ALF. He doesn't drive the entire series' plot, but often drives a whole episode.
 * The '90s Biker Mice From Mars series had this with the Planet Looting plutarkians. One example being Limburger.
 * Parodied to no end in Invader Zim, where the alien doesn't do much to hide his identity, yet it somehow works.
 * In The Fairly OddParents, alien prince Mark Chang flees to Earth to escape an Arranged Marriage.