Animorphs
Animorphs is a science fantasy series of children's books written by Katherine Applegate and her husband Michael Grant, writing together under the name K. A. Applegate, and published by Scholastic. It is told in first person, with all six main characters taking turns narrating the books through their own perspectives. Horror, war, dehumanization, sanity, morality, innocence, leadership, freedom, family, and growing up are the core themes of the series.

Published between June 1996 and May 2001, the series consists of 54 books and includes ten companion books, eight of which fit into the series' continuity (the Animorphs Chronicles and Megamorphs books) and two that are gamebooks not fitting into the continuity (the Alternamorphs books). The series was originally conceived as a three-part series called The Changelings, in which Jake was named Matt, and his little brother Joseph took the place of Cassie. The books were also adapted into a television series of the same name on Nickelodeon, YTV and Global Television Network from 1998 to 1999.

Plot summary
The story revolves around five humans: Jake, Marco, Cassie, Rachel and Tobias, and one alien, Aximili-Esgarrouth-Isthill (nicknamed Ax), who obtain the ability to transform into any animal they touch. Naming themselves "Animorphs" (a portmanteau of "animal morphers"), they use their ability to battle a secret alien infiltration of Earth by a parasitic race of aliens resembling large slugs called Yeerks, that can take any living creatures as a host by entering and merging with their brain through the ear canal. The Animorphs fight as a guerilla force against the Yeerks who are led by Visser Three.

Throughout the series, the Animorphs carefully protect their identities; the Yeerks assume that the Animorphs are a strike force sent by the Andalites, the alien race to which Ax belongs that created the transformation technology, to prevent them from conquering Earth. To protect their families from Yeerk reprisals, the Animorphs maintain this façade.

Though the Animorphs can assume the form of any animal they touch to acquire the DNA, there are several limitations to the ability. The most vital is that they cannot stay in animal form for more than two hours, or they will be unable to return to human form and the morphs become permanent. Others include having to de-morph back to human in between morphs, only tight clothing being able to be carried over with a morph, and having to consistently maintain concentration during a morph to prevent the animal's natural instincts from overwhelming the human intellect. A benefit to morphing is that it allows the team to heal any superficial, non-genetic injury, sustained as a human or in a morph. Also, while in morph, they can telepathically communicate with anyone nearby.

Development
In an interview with Publishers Weekly, Applegate talked about the source of inspiration and realization for the Animorphs series: "I grew up loving animals and lived with the usual suburban menagerie of dogs, cats and gerbils", she said. "I really wanted to find a way to get kids into the heads of various species and decided that a science-fiction premise was the way to do this." Applegate tried to accurately depict the various animals, and did research such as visiting "a raptor center where they rehabilitate injured birds". "When Tobias becomes a hawk, I want the reader to see the world as a hawk might see it—to soar on the warm breezes and hurtle toward the ground to make a kill," she said.

To develop the characters for Animorphs, Applegate would go through teenage magazines such as YM and Seventeen (both of which are referenced in the books when describing Rachel), cutting out pictures and piecing them together to get an idea of what sort of children the Animorphs would look like. Applegate stated in an interview online that many of the names for her alien creatures, races, and locations are actually scrambled names of local street signs or companies that she happens to notice. For instance, the word nothlit was derived from the hotel name Hilton. According to the Anibase, Applegate did not make up the titles for the Animorphs books: it was up to the Scholastic editors to create the titles for the books based on the outlines provided by the author, having to select a word that not only fit the book's storyline, but sounded good with the characteristic "The" preface. One of the author's favorite books, The Lord of the Rings, lent several words and images to Animorphs: the elvish word for Orc, "yrch", became Yeerk; the flaming red Eye of Sauron inspired the Crayak, and Ax's middle name, "Esgarrouth", is based on a town in the books called Esgaroth. The human name of Ax's brother, Elfangor, is Alan Fangor and his last name is in reference to the Fangor region or Fangorn Forest. Also there was a minor reference to Gondor, in the form of a fictional company named "Gondor Industries" in the 14th book. (It may also be significant that Visser Three's host is named Alloran, a rough homonym of Gandalf's Valinorean name "Olórin", and that one of the minor alien races is called "the Five", which is also a term used in The Lord of the Rings for the Istari.)
Applegate's writing was inspired by her family. All books after The Unknown were dedicated to Applegate's child, Jake, as well as her husband and co-writer, Michael. Her daughter was born premature in 1997, and Applegate worked on the Animorphs series at night, in the lobby of the hospital where she was in Neonatal Intensive Care (NIC).

Characters

Animorphs
Jake: Jake Berenson is the leader of Animorphs. Despite being a natural at it, he's very reluctant to lead the team, though he ultimately accepts the role. The war takes on a deeper meaning for Jake when he finds out that his older brother Tom has been infested by a Yeerk, it is also one of the only things that holds him together throughout the series; if not for that he would probably crack from all the pressure and life-taking decisions. He is the only member of the team to have some relationship with all of the other human members prior to the war: Jake and Rachel are cousins. He and Marco have been best friends since early childhood. He and Cassie have always had an attraction to each other.  He is one of the few people who acknowledges Tobias and treats him with kindness.

Rachel: Rachel has the most bloodthirsty nature in the group, earning her the nickname of "Xena, Warrior Princess". She is also good in gymnastics and has an interest in fashion. She is Jake's cousin and is Cassie's best friend, although her warrior nature often conflicts with Cassie's pacifistic mindset. She and Tobias develop an attraction for one another during the series.

Tobias: Tobias has low self-esteem and was often bullied in school before becoming an Animorph.  With both his parents dead, he lives with his aunt and uncle who share custody of him. Tobias becomes trapped in a red-tailed hawk morph in the first book.  During the series, he regains the power to morph, but his natural form is now as a hawk. Even though Rachel wants him to morph back to human and stay for more than the two hour time, but that would mean he would be out of the fight, so he remains as a red-tailed hawk. He admires Jake because he has always shown him respect and kindness when no one else would. He also develops a close friendship with Ax.

Cassie: Cassie lives on a farm with her parents, both of whom are veterinarians. She is the most knowledgeable about animals, and she is also an environmentalist. She is best friends with Rachel, although their personalities and styles are polar opposites. She loves nature and animals; that is why Marco always calls her a "tree-hugger". Cassie is also an estreen, the Andalite term for someone who is naturally talented at morphing.

Marco: Marco is the comic relief character in the series and Jake's best friend. He is also the one who coined the term "Animorphs."  Marco is very pragmatic, often proposing the most direct solution to a given problem, no matter if it involves some sacrifices. He lives with his father, who is depressed over his wife's "death" in a boating accident. At first, he doesn't want to fight because he fears that his father can't survive without him.  He changes his mind when he finds out his mother is still alive and the host body for Visser One, the leader of the Yeerk invasion.

Aximili-Esgarrouth-Isthill (Ax): Aximili-Esgarrouth-Isthill is an Andalite who is thought to be the only survivor of a bloody battle that took place in outer space. His ship has crash landed on Earth. He is the younger brother of Prince Elfangor-Sirinial-Shamtul, who gave the Animorphs their morphing ability and died shortly after. He considers Jake his 'prince' (similar to an earth military commander) and has a strong friendship with Tobias.  The other Animorphs call him "Ax" because it is easier to pronounce.

Other main characters
Visser Three: Born as Esplin 9466 Primary and later known as Visser One, he is the leader of the Yeerk forces on Earth and the main antagonist of the series. He inherited the planet from Edriss 562, who was Visser One at the time. Sadistic and cruel, with a penchant for torture, he almost exclusively inhabits the body of a fully-grown Andalite warrior, Alloran-Semitur-Corrass, and is the only Yeerk to have an Andalite host. He also has the Andalite ability to morph, and is shown throughout the series to have acquired some exceptionally powerful morphs from many different planets. He later gets promoted to Visser One.

Ellimist: A seemingly all powerful, God-like being who intermittently helps the Animorphs, including giving Tobias the ability to morph again. He tries to refrain from direct involvement in the war to avoid antagonizing his evil counterpart Crayak because combat between the two of them causes destruction on a galactic scale. He subtly orchestrated many of the key events of the series. In The Ellimist Chronicles, it is revealed that he was once a simple member of an avian race who, through a number of experiences, became a drastically different being.

Crayak: The arch-enemy of the Ellimist who seeks absolute control over all intelligent life. He develops a personal vendetta against Jake after Jake ruins his soldier species, the Howlers. The Ellimist claimed that he originated from another galaxy and was driven out by another power even greater than he was; The Ellimist Chronicles details his early conflicts with the Ellimist prior to their evolution into beings of seemingly limitless power.

Secondary characters
Andalites:
 Aldrea-Iskillion-Falan - Aldrea is the daughter of Prince Seerow, a main character in Animorphs #34: The Prophecy and the heroine in The Hork-Bajir Chronicles. She has a strong personality mixed with a desire to do the right thing, not necessarily what is easy. Born an Andalite, her father Seerow gave the Yeerks technology that they used to leave their home planet and begin their quest to dominate the universe. Her family was eventually sent to the Hork-Bajir home planet, where she bonded with an unusually intelligent Hork-Bajir named Dak Hamee. Aldrea obtained the ability to morph shortly after the technology was developed, and used it to fight the Yeerk invasion of the Hork-Bajir world after her family was murdered. She and Dak led the resistance, and a strong bond developed between the two of them. She eventually, though accidentally, became a Hork-Bajir female permanently, and she and Dak fell deeply in love, married and had a son named Seerow. Despite their brave resistance efforts, they were both eventually killed and their son was infested by the Yeerks, though his line later gave rise to the leaders of the free Hork-Bajir on Earth. A copy of Aldrea's memories and personality made before her death was later temporarily implanted in Cassie in order to gain access to a cache of Yeerk weapons stolen and hidden by Aldrea and the Hork-Bajir resistance on the Hork-Bajir homeworld.
 Alloran-Semitur-Corrass - Andalite host body of Visser Three. Alloran was an officer under Prince Seerow, who relieved his superior of duty after a force of Yeerks stole Andalite ships and left their planet thanks to technology provided to them by Seerow. Years later, having been promoted to the rank of War Prince, Alloran was placed in command of the Andalite forces sent to fight the Yeerk invasion of the Hork-Bajir homeworld. Unfortunately, the desperate situation on the planet led Alloran to decide that wiping out the Hork-Bajir with a virus was a better alternative than allowing them to be enslaved, and countless members of the species died as a result. Alloran was subsequently disgraced, and ended up serving on the crew of the same ship that Elfangor was assigned to as a junior officer. A mission that the pair went on with Elfangor's peer Arbron and a pair of humans led to Alloran's infestation by the Yeerk who would become Visser Three, who used Alloran's morphing abilities for many vile purposes throughout the years. After an attempt by Ax to kill Visser Three, Alloran was briefly left free to communicate with him, and encouraged him to continue the fight. Eventually he was freed from his infestation.
 Elfangor-Sirinial-Shamtul - Elfangor is the first alien the Animorphs meet in the series. He gives them the power to morph minutes before his death.  His adventures are highlighted in the book called The Andalite Chronicles, which takes place before the main Animorphs series. He was a war prince, and the older brother of Aximili-Esgarrouth-Isthill. He was the sworn enemy of Visser Three, and - due to time spent as a human - the father of Tobias.
Auxiliary Animorphs: These were teenagers recruited from hospitals and rehabilitation centers where they were being treated for chronic diseases or learning to live with disabilities.  The ability to morph restored some of the teenagers' bodies to perfect health, but failed to when the diseases or conditions were genetic.
David: A boy who joins the Animorphs after learning their secret. The Animorphs trust him at first, giving him the ability to morph and fight with them rather than kill him, but he betrays them.  Ultimately, they are forced to trick him into morphing into a rat and trap him permanently in the morph, leaving him stranded on an island.  He returns later in the series, eventually asking Rachel to kill him.  Whether Rachel kills David or not has never been established.
Drode: The Drode is an alien creature, described as being similar to a very dark purple dinosaur with wrinkled, pruny skin, and an oddly humanoid face. Little is known of the Drode, but it serves Crayak and usually shows up when he does.
Erek King: Erek King is a member of the Chee, a pacifistic android race created by an alien race called the Pemalites. After the destruction of the Pemalites at the hands of the Howlers, Erek, along with the rest of the Chee, escaped to Earth.  They have lived on the planet for thousands of years, using incredibly advanced holographic technology to pass as human.
Hork-Bajir:
 Toby Hamee - Leader of the Free Hork-Bajir on Earth; a Seer or exceptionally intelligent member of the Hork-Bajir race.
 Jara Hamee - Toby's father and the descendant of Dak Hamee, a Hork-Bajir Seer, and Aldrea, an Andalite-turned-Hork-Bajir who led the Hork-Bajir resistance against the Yeerks.  He was killed in action during the final battle.
 Ket Halpak - Toby's mother and Jara Hamee's wife.
Howlers: A race of killers artificially created by Crayak and deployed as shock troopers against the inhabitants of various worlds. Seven of them were selected by Crayak to be pitted against the Animorphs and Erek the Chee in a contest between the Ellimist and Crayak concerning the fate of an offshoot of the Yeerk race who had found an alternative means of living. Jake eventually discovered that the Howlers were children, too young to understand what they were doing and seeing it all as a game, and the Animorphs managed to infect their collective memory with a flashback of Jake and Cassie kissing. The Howlers were thus ruined as a soldier race.
The One: An alien being who assimilates other beings; he makes a deal with the defeated Yeerks.  He is the final antagonist of the series.
Tom Berenson: Jake's older brother, a Controller. His original Yeerk is actually promoted and meant to receive another host early in the series, but ends up in Jake instead; it subsequently dies, while Tom receives another Yeerk. This Yeerk eventually stole the cube that provided the morphing power to the Animorphs and delivered it to his fellow Yeerks, and was later subsequently killed by Rachel.
Hedrick Chapman: Vice-principal of the Animorphs' school and a Controller.
Yeerks: The main antagonists, described as small, slug-like parasites that enter various organisms' brains to control their behavior. They are dependent upon Kandrona rays to survive, and must leave their hosts every three days in order to enter a Yeerk pool where they may absorb these rays.
 Edriss 562 - Edriss 562 is a Yeerk that controls Marco's mother Eva. For most of the series, her rank is Visser One. She is the subject of the novel Visser, which describes her rise to and dramatic fall from power. She is the highest ranking of all the Yeerks in their military, and is only surpassed in importance by the Council of Thirteen.

Characters' age
Throughout the publication of the series, there was some dispute about the exact ages of the Animorphs at the time they obtained the ability to morph. However, with the help of various hints in the course of the series, many fans guessed their ages to be approximately thirteen to fourteen (with thirteen being the more likely) at the start. For example, at the beginning of #1: The Invasion, Jake mentions having tried out for his junior high basketball team and not making it.  This puts Jake, Rachel, Tobias, Cassie, and Marco, at the very least, around the age of eleven to fourteen, as junior high (or middle school) in the United States is generally grades six/seven through eight.  However, as Marco describes them as "idiot teenagers with a death wish" in the first book, it is very likely that some or most of them are older than twelve. This is also supported in #2: The Visitor, when Rachel looks at a photo "taken a couple of years ago" of her and Melissa Chapman, taken on Melissa's "twelfth birthday, or some birthday." Although Rachel cannot remember what birthday it was, this supports the idea that the Animorphs are either thirteen or fourteen. In #22: The Solution, Rachel states that Jake is "not even in high school." This suggests that none of the characters are, because they eat lunch together in school earlier in that book.  #26: The Attack definitively confirms the characters attending middle school when Jake, in the first chapter, says that he is "a middle-school kid" in his narration. Also, in #41: The Familiar, Jake wakes up one morning as a twenty-five-year-old, and in the preview for that book in the previous one, it says he sleeps for a decade, suggesting that his age was fifteen before his journey into the future.

#52: The Sacrifice provided a clear answer as to the question of grade level. In the first chapter of the book, Ax says in his narration that Jake, Rachel, Cassie, and Marco are all currently of age to be attending high school.  This puts the Animorphs' ages as anywhere between fourteen and eighteen (as high-school students in the United States typically fall within this age range). However, the publication of #53: The Answer offered a definite answer to the question of age. Jake says outright at the start of the second chapter that he is sixteen, started the war when he was thirteen, and has been fighting the war for over three years. Marco also states in chapter eight of the final book that Jake is sixteen. Throughout the course of the final book, two or three more years passed. Cassie mentions that she is nineteen in her final scene of the book, although the other characters' ages are never explicitly confirmed. In the end, the characters are either nineteen or twenty years old, depending on how long they had been in space just before the series' conclusion.

Publication
Each book in the series revolved around a given event during the war waged between the Animorphs and the invading Yeerks.  Within a year and a half after the first book was published, the series had close to ten million copies in print, with Scholastic claiming a "stronger initial sell-in," than any of its other series up to that time. The series debut was preceded by a large marketing campaign which included posters on buildings, giveaway items in bookstores, and ads on Nickelodeon TV.

American editions
In the United States, the books were most popular as A5-sized paperback volumes, and were usually between 150 and 200 pages long, divided into just under thirty chapters.

The front covers featured images of the narrating Animorph undergoing the various stages of one of the morphs from the story, with a few exceptions (noted in each book's article). Behind the morphing character were images of clouds and skies, which became more colorful and elaborate as the series progressed. All the covers of the regular series books had a small cutout over part of the full morph's anatomy, revealing a computer-generated illustration on the first page, which was printed on glossy paper. The illustration shared the image of the full morph with the front cover, but placed within an environment from the story. The book spines repeated the narrating character's face from the front cover, and the spine color changed with every new episode, resulting in a very colorful collection when viewed from any angle. A small excerpt from one of the book's chapters was printed on the inside of every front cover.
As of the eighth book, The Alien, the Animorphs logo, the author's name, and the book's title were printed in glossy, metallic-look ink, rather than the flat colors that had been used for the first seven books. In addition, the author's name and book title were surrounded by solid black rectangles. The majority of the books in the series were printed only in "metallic-ink editions". All further reprintings of the first seven books had this treatment applied to them as well.

The books in the series' final arc, beginning with the 45th book, The Revelation had yet another treatment applied to the cover, a variation on the new metallic style; the change affected only the main 'Animorphs' logo: instead of consisting of white letters superimposed on a metallic, colored background, the last ten books featured a logo with colored letters over a dark grey background, in contrast with the white logo background from the series' "opening arc". The final book, #54 The Beginning had a unique cover style, with the logo consisting of a glowing outline.

Every book featured an introduction to the series on the back cover, in the voice of Jake, one of the Animorphs.

We can't tell you who we are. Or where we live. It's too risky, and we've got to be careful. Really careful. So we don't trust anyone. Because if they find us... well, we just won't let them find us.

The thing you should know is that everyone is in really big trouble. Yeah. Even you.

As of book 51, The Absolute, the introduction read as follows:

Here's the deal these days: They know exactly who we are. They know exactly where we live. We've got a few secrets left, and we're gonna use them. But just know that the end is coming. And we don't know how much longer we can do this. How much longer can we fight.

What about you? Where will you be when it ends? Think about it. Think hard. Because the countdown has already begun...

In addition to this text, each book also carried an introduction, or teaser of sorts, to its own storyline.

Another interesting feature of the books was a flipbook composed of the bottom right-hand corners of all of the book's pages. A step of the cover morph was printed on each page, less than an inch tall, in black-and-white. When the pages were flipped from front to back, the narrating Animorph could be seen morphing into the animal.

International editions
The Animorphs series was printed in over twenty-five languages and other English-language markets, and the books in those countries sometimes had different designs, layouts, cover quotes, and even different cover morphs, as is the case for the fifth book, The Predator, whose UK edition showed Marco morphing into a lobster, in contrast to the American edition's gorilla morph. Japanese-language covers were hand-drawn; The Invasion showed Jake morphing into his dog Homer, a morph that was featured on the cover of The Threat in the American editions. Gallimard Jeunesse is the French publisher and Tammi is the Finnish publisher. The German publisher, Ravensburger, has also published some of the volumes as audio plays.

Animorph Classics
In 2010, Scholastic announced plans to re-release the series with new lenticular covers and updated pop culture references. The re-release lasted from May 2011 to September 2012, ending after #8: The Alien due to tepid sales.

Books 

The series consists of 54 books and includes ten companion books, eight of which fit into the series' continuity (the Animorphs Chronicles and Megamorphs books) and two that are gamebooks not fitting into the continuity (the Alternamorphs books).

Ghostwriters
Many of the novels from the #25-#52 range were written by ghostwriters. Typically, K. A. Applegate would write a detailed outline for each book, and a ghostwriter, usually one of Applegate's former editors or writing protégés, would spend a month or two writing the actual novel. After this, Applegate, and later her series editor, Tonya Alicia Martin, would edit the book to make it fit in with the series' tight continuity. Ghostwriters are credited for their help in the book's dedication page: "The author would like to thank [ghostwriter name] for his/her help in preparing this manuscript."

The only books in this range fully written by Applegate herself after #26: The Attack are #32: The Separation, #53: The Answer, #54: The Beginning and all of the Megamorphs and Chronicles books.

The following books in the series were ghostwritten:

 #25: The Extreme - Jeffrey Zeuhlke
 #27: The Exposed - Laura Battyanyi-Weiss (Due to an editorial oversight, Battyanyi-Weiss was uncredited for this book.)
 #28: The Experiment - Amy Garvey
 #29: The Sickness - Melinda Metz
 #30: The Reunion - Elise Donner 
 #31: The Conspiracy - Laura Battyanyi-Weiss
 #33: The Illusion - Ellen Geroux
 #34: The Prophecy - Melinda Metz
 #35: The Proposal - Jeffrey Zeuhlke
 #36: The Mutation - Erica Bobone
 #37: The Weakness - Elise Smith
 #38: The Arrival - Kimberly Morris
 #39: The Hidden - Laura Battyanyi-Weiss
 #40: The Other - Gina Gascone

 #41: The Familiar - Ellen Geroux
 #42: The Journey - Emily Costello
 #43: The Test - Ellen Geroux
 #44: The Unexpected - Lisa Harkrader
 #45: The Revelation - Ellen Geroux
 #46: The Deception - Elise Donner
 #47: The Resistance - Ellen Geroux
 #48: The Return - Kimberly Morris (Due to an editorial oversight, Lisa Harkrader was mistakenly credited for this book.)
 #49: The Diversion - Lisa Harkrader
 #50: The Ultimate - Kimberly Morris
 #51: The Absolute - Lisa Harkrader
 #52: The Sacrifice - Kimberly Morris
 Alternamorphs#1 - Tonya Alicia Martin (Applegate's series editor)
 Alternamorphs#2 - Emily Costello

Applegate originally intended to write every Animorphs book herself. However, due to many contributing factors—such as the birth of her child and the difficulties involved in writing Everworld (which was originally intended to be mostly ghostwritten, like Applegate's third Scholastic series Remnants), she ended up having a large number of the books ghostwritten.

Toys
The Animorphs toy line was introduced in 1999 by Hasbro. They were marketed as part of the Transformers series, despite there being no in-universe connection between the two franchises. However, the Animorphs toys were commercially unsuccessful and the toy line was soon cancelled. After the cancellation, several toys planned to be part of the Animorphs line were slightly remodeled and released as part of the Beast Wars Mutants line.

Adaptations

Television series 
A television series of the same name ran from September 1998 to March 2000 in the United States and Canada. Animorphs comprised 26 episodes over two seasons, which aired on YTV (first season) and Global (second season) in Canada and Nickelodeon in the United States.

Film 
In September 2015, several film websites began reporting rumors that Universal Pictures had plans to adapt the book series into a film, based on a report by the film website The Tracking Board. The site also claimed that Universal would be working with Silvertongue Films, a production house launched to develop Scholastic books into feature films, and that Deborah Forte would be producing.

Video games 
There are overall three different video games based on the book series, all released initially in 2000.

Animorphs: Know the Secret is an action-adventure game, released for Microsoft Windows. Developed by Gigawatt Studios and published by Infogrames, it lets the player to switch control between four of the Animorphs (Cassie, Jake, Marco and Rachel). They will explore different environments searching for clues as to why the activities of Yeerks have gone down. The Animorphs have access to a number of specific forms they can change into, which can increase by finding more animals on the way.

Animorphs: Shattered Reality is a platform video game, released for PlayStation. Developed by SingleTrac, it revolves around four of the Animorphs trying to find pieces of the Continuum Crystal before Visser Three does.

A Game Boy Color game was also published by Ubisoft.