Horror, war, imperialism, dehumanization, sanity, morality, innocence, leadership, freedom, family, and growing up are the core themes of the series.

[5] The story revolves around five teenage humans: Jake, Marco, Cassie, Rachel, 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"),[6] 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 creature as a host by entering and merging with their brain through the ear canal.

The Animorphs fight as a guerrilla force against the Yeerks who are led by Visser Three, who uses an Andalite (a centaur-like race of aliens, which Ax belongs to) as a host.

[citation needed] 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".

In another interview, Applegate stated that she originally wanted the alien Andalite to have more standard and familiar forms, but was told by Scholastic to be more creative with the designs, which led to her giving them such a distinctive look.

One of the author's favorite books, The Lord of the Rings, lent several words and images to Animorphs: the Sindarin plural 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.

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).

Mattingly had never done a morph before, but ordered a copy of Elastic Reality and completed a sample within two days, which eventually became the cover for book #3: The Encounter.

Each of Mattingly's covers took two weeks to create, from pencil sketch to finished digital render and required painting, since the Elastic Reality program produced smeared or stretched material.

Since Scholastic wanted the covers to feature photorealistic humans, model reference photos were taken by photographer Addie Passen at her Union Square studio.

[14] 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.

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.

#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.

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.

[16] The series debut was preceded by a large marketing campaign which included posters on buildings, giveaway items in bookstores, and ads on Nickelodeon TV.

[16] 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.

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 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.

In 2010, Scholastic announced plans to re-release the series with new lenticular covers and updated pop culture references.

[22][23] In June 2020, it was formally announced that an Animorphs film would be produced by Scholastic Entertainment, and Picturestart, the latter run by Erik Feig and Lucy Kitada.

[25] Grant later clarified his remark, wishing the producers the best and noting he and Applegate simply were not being involved enough in the production process.

[27] Developed by Gigawatt Studios and published by Infogrames,[27] it lets the player to switch control between four of the Animorphs (Cassie, Jake, Marco and Rachel).

MacLeod Andrews narrates Jake (and Elfangor, for The Andalite Chronicles), Emily Ellet for Rachel, Michael Crouch for Tobias, Sisi Aisha Johnson for Cassie, Ramón de Ocampo for Marco, and Adam Verner for Ax.

Mark Bramhall narrates as Jake's Civil War ancestor Isaiah Fitzhenry for parts of book #47, The Resistance.