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 morph 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.

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 kids 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 son, Jake, as well as her husband and co-writer, Michael. Her son was born premature in 1997, and she worked on the Animorphs series at night, in the lobby of the hospital where he was in Neonatal Intensive Care (NIC).

The names given here are the ones used throughout the majority of the series; in the last few books, some character's full names are revealed.

Animorphs

• Jake

Jake is the leader of Animorphs. He was reluctant to take the role at first, but quickly changed his mind when he found out that his older brother Tom has been infested by a Yeerk. He is Rachel's cousin. He is also best friends with Marco since early childhood, and he is romantically involved with Cassie.

• Marco

Marco is the comic relief character in the series. He was also the one who coined the term "Animorphs", and he has a good sense of strategy.

• Tobias

Tobias has low self-esteem and was often bullied in school before becoming an Animorph. His parents were believed to be dead, and he lived with his aunt and uncle. Tobias was trapped in a red-tailed hawk body in the first book and he stays as a hawk throughout the series, although he eventually regains the ability to morph, including the ability to morph back into his human form. He has a romantic relationship with Rachel, and he admires Jake because he once stopped some bullies from beating him up.

• Cassie

Cassie lives on a farm with her parents, who are both veterinarians. She is the most knowledgeable about animals and she is also an environmentalist. She is best friends with Rachel, although their personality and style are polar opposites. Cassie maintains a casual romantic relationship with Jake throughout the series although they end up parting ways.

• 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 a lingering obsession with fashion. She is Jake's cousin and is Cassie's best friend, although her warrior nature often conflicts with Cassie's pacifist attitude.

• Aximili-Esgarrouth-Isthill

The other animorphs call him Ax because they claim it difficult to pronounce his full name. Ax is an alien, an andalite who crash landed on earth. He is the younger brother of Elfangor, who gave the Animorphs their morphing ability. He considers Jake his prince and he has a strong friendship with Tobias, especially after discovering that Tobias is the son of his older brother Elfangor's human form.

Other main characters

• Ellimist

He was the one who chose most of the Animorphs. He gave Elfangor a second chance at being an Andalite (the Andalite Chronicles) and is one of the main protagonists who fight Crayak. He chose three of the Animorphs: Tobias, Cassie, and Marco.

• Crayak

The main villain after the Yeerks who fights out of time-space against the Ellimist and the Animorphs with the help of the Drode.He has a very large disliking of Jake because he jammed a memory into one of his howlers.

Secondary characters

• Aldrea-Iskillion-Falan
• Alloran-Semitur-Corrass
• Arbron
• Auxiliary Animorphs
• David
• Drode
• Elfangor-Sirinial-Shamtul
• Erek King
• Toby Hamee
• Visser One
• Visser Three
• Jara Hamee
• Ket Halpak
• Tom Berenson
• Hedrick Chapman

Minor characters

• Andalites
• Hork-Bajir
• Humans
• Ketrans
• Yeerks
• Other minor characters

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 13-14 (with 13 being the more likely) at the start. For example, at the beginning of the first book, The Invasion, Jake mentions having tried out for his junior high basketball team and not making the team; this puts Jake, Cassie, Marco, Rachel, and Tobias, at the very least, around the age of 11-14 as junior high (or middle school) in the United States is generally grade 6-8. However, as Marco describes them as "idiot teenagers with a death wish" in the first book, it's very likely that some or most of them are older than 12. This is also supported in book two, 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 13 or 14. 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. And in #41: The Familiar, Jake wakes up one morning as a 25-year-old, and in the preview for that book in the previous one, it says he sleeps for a decade, suggesting his age was 15 before his journey into the future.

The publication of book #53 offered a definite answer to the question of age. Jake says outright at the start of chapter 2 in #53 that he is 16, started the war when he was 13, and has been fighting the war for over three years. Marco also states in chapter 8 of book #54 that Jake is sixteen. Throughout the course of book #54, 2 or 3 more years passed. Cassie mentions that she is 19 in her final scene of book #54, although the other characters' ages are never explicitly confirmed. In the end, the characters are either 19 or 20 years old depending on how long they had been in space just before the series' conclusion.

Each book in the series revolved around a given event during the war waged between the Animorphs and the invading Yeerks, the first book detailing how the Animorphs came to have their power. 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 that they're planning to re-release the series with new lenticular covers, and updated old references in the books. The first six books were planned to be released every few months starting from May 2011.

As of May 2012, they have released the first six books of the series and announced the future relaunch of book 7 and 8.

The common text that each book has on its back has been shortened to:

We can't tell you who we are. Or where we live. It's too risky, and we've got to be careful. But everyone is in danger. Yeah. Even you.

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 son 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.

On December 11th, 2012, it was discovered that Sony Pictures had registered four domain addresses that may or may not allude to their involvement in an Animorphs movie. There is yet to be any official word, however all four domains redirect to Sony Pictures website. The URLs include:

animorphs.net animorphs-movie.net animorphsmovie.com animorphs-movie.com

• Animorphs (TV series)
• List of species (Animorphs)
• List of Animorphs books
• Chronological list of Animorphs books
• Andalite terminology
• Hork-Bajir terminology
• Animorphs (toy)
• Michael Grant
• David B. Mattingly
• Animagus, a similar shape-shifting ability in the Harry Potter books by J.K. Rowling
• "Shifters" are a race of humans in the The Southern Vampire Mysteries novels by Charlaine Harris. They transform into animals in the same way as Animorphs - by being close to them and mimicking their form.