Unlike its predecessor Imaginaerum, which has themes of imagination and fantasy, Endless Forms Most Beautiful addresses science and reason.

Focusing on the evolutionary theories of Charles Darwin and Richard Dawkins, the latter part of the album cites passages from their books.

On 28 September, before a show in Denver, Olzon became ill and was replaced by Elize Ryd and Alissa White-Gluz (vocalists for Amaranthe and The Agonist, respectively).

[6] The album was introduced in May 2014, when Holopainen posted on his website that it would probably be ready at the end of January 2015 and demos would be circulated beginning in July 2015.

[11] Like every Nightwish album, Endless Forms Most Beautiful was primarily written by Tuomas Holopainen with (as since 2002) singer and bassist Marko Hietala secondary songwriter.

[12] About the songs after recording a first demo, Holopainen said: "It's still too early to analyze the material more closely, but the album will once again explore all the ends of the spectrum, bringing the very best out of the newcomers Floor [Jansen] and Troy [Donockley].

"[22] Holopainen called it heavier than its two predecessors, citing "Weak Fantasy", "Yours Is an Empty Hope" and the title song,[11] with more prominent bass and guitars.

[13][19] The band began rehearsing and recording over the following months in Eno, Finland, in a rented "cabin" in an isolated area of snow and trees they called "summer camp".

"[16] On 6 August 2014, drummer Jukka Nevalainen announced that he would not be part of the upcoming album and tour due to severe insomnia.

It would be the first Nightwish album without him:[26] "During the last month, it has become evident for me that due to my insomnia, I can't fulfill my duties as a musician in the way that I'd want and with the precision that the music deserves.

Nevalainen said that his good friend Kai Hahto, drummer for melodic death metal bands Wintersun and Swallow the Sun and a longtime Nightwish drum technician, would be his replacement on the album.

"[15] On 28 September Nightwish reported that they were leaving the "summer camp" after three months of work for London to record the orchestra, choirs, and percussion.

Jansen, who had never worked with an orchestra before, said that she was "stunned by the professionalism of the players, the conductor James Shearman, the studio engineers and by the amazing arrangements Pip made".

[36] Holopainen wrote a letter to Dawkins, who answered two weeks later by email that he had never heard of the band;[17] he became interested after listening to some of their songs on the Internet.

[40] Its lyrics, which criticize how some religions restrict lives,[41] were also co-written by Hietala (who contributed to an originally-instrumental (and acoustic guitar-centered[42]) part of the song which the band felt was too long).

[40] Although he was initially uncertain about recording "Yours Is an Empty Hope" (co-written by Hietala and also called one of the album's heaviest songs),[40] Holopainen found its subject matter "inspirational".

"My Walden", a Celtic song with a prominent contribution from Donockley, was considered by Holopainen a continuation of "I Want My Tears Back" from Imaginaerum.

"Endless Forms Most Beautiful" is inspired by Dawkins' book, The Ancestor's Tale,[40] and "Edema Ruh" refers to the group of traveling minstrels in Patrick Rothfuss's The Name of the Wind;[40] it was the first song written for the album.

[10][45] "Alpenglow" has been called the "ultimate Nightwish song" by Holopainen, who referred to it as "a nice little interlude" with "a catchy chorus and toxic guitar riff".

[40] He also stated that it is the result of him wanting to write a love song for the album with the particular aim to find a new angle for storytelling, contrasting that there are enough "awfulnesses" like Bon Jovi’s "Always" already in existence.

According to Holopainen, the most important secret to approach universal themes such as wonder, longing or death is to always find new ways to translate the respective emotions and stories into music.

After a suggestion by Donockley, Holopainen decided to add only distant voices and a children's choir[11][40] and said that it works as an intermission between the first part of the album and the final track.

The song features short excerpts from Dies Irae, Minuet in G major by Christian Petzold, Toccata and Fugue in D minor, BWV 565 by Johann Sebastian Bach and Enter Sandman by Metallica, accompanied with a Tibetan chant sung by male parts of the choir which are probably references to the evolution of music and arts as part of evolution of the human race.

According to Don Lawson of The Guardian, Nightwish's use of a less-fanciful theme introduced elements which "result in their most ambitious and assured record yet" and he praised Holopainen's songwriting.

Lawson wrote that Jansen "strikes a fine balance between operatic acrobatics and straightforward, soulful restraint, most notably on twinkling earworm 'Élan'.

"[52] Metal Storm called Endless Forms Most Beautiful "a sweet album of awesome songs" which "doesn't try to be some over the top attempt to grandly announce a new era for the band" and "a smart move".

[53] Solomon Encina called the album the band's most accomplished effort in Metal Injection, although he found "Élan", "Alpenglow" and "Endless Forms Most Beautiful" repetitive and stale.

[60] Craig Hartranft, founder of Dangerdog Music Reviews, called Endless Forms Most Beautiful "pretty darn terrific".

[51] MusicReviewRadar wrote: "[I]n Endless Forms Most Beautiful [Nightwish] evolved to their own personal and original style" leaving him "craving for their next album".

A new crab species, called the Tanidromites nightwishorum was discovered by curator of palaeontology, Dr Adiel A. Klompmaker on 10 July 2020, and was named in honor of the band in particular for the album.