It was originally serialized in Shogakukan's seinen manga magazine Big Comic Spirits from 1999 to 2006, with the 249 chapters published into 22 tankōbon volumes.

The series makes many references to a number of manga and anime from the 1960s–1970s, as well as to classic rock music, its title being taken from T. Rex's song "20th Century Boy".

20th Century Boys has received critical acclaim and has 36 million copies in circulation, making it one of the best-selling manga series of all time.

In 1969, young boys Kenji, Otcho, Yoshitsune and Maruo build, in an empty field, a hideout they call their secret base, in which they and their friends can get together to share manga and stolen pornographic magazines and listen to a radio.

In the late 1990s, Kenji is a convenience store owner, finding solace in his childhood adventures as he takes care of his baby niece Kanna and his mother.

From this event, the members of the Friendship Democratic Party (友民党, Yūmintō) gain widespread political popularity and power by presenting a vaccine that counters the virus, and thus take all the credit for saving the world.

The final portion of the story takes place in a newly remodeled Japan, under the "Era of Friend", who has instituted numerous bizarre changes, including the establishment of an Earth Defense Force, reputedly to protect Earth from an imminent alien invasion, exiling those without vaccinations, and forbidding travel across regions, under penalty of death.

On the day he gave his editor the manuscript for the final chapter of Happy!, Naoki Urasawa was relaxing in the bath when he heard a speech on television by someone from the United Nations say "Without them, we would not have been able to reach the 21st Century..." and wondered "Who's 'them'?

[24] A few weeks before the September 11 attacks, Urasawa turned in a manuscript for 20th Century Boys where two giant robots fight and destroy buildings in Shinjuku.

But after the attacks, the artist could not bring himself to illustrate that scene and created a chapter almost entirely devoted to Kenji singing a song, in order to express how he felt.

Although he was briefly hospitalized for exhaustion at this point, Monster ended in 2001 and Urasawa began writing another series simultaneous to 20th Century Boys in 2003 with Pluto.

[28] Written and illustrated by Naoki Urasawa, 20th Century Boys was originally serialized in Shogakukan's seinen manga magazine Big Comic Spirits from October 4, 1999,[a] to April 24, 2006.

[35] A sequel that concludes the story, titled 21st Century Boys, was serialized in the same magazine from December 25, 2006,[c] to July 14, 2007;[38] the 16 chapters were released into two volumes on May 30 and September 28, 2007.

[51] The first movie's premiere was held in Paris on August 19, 2008, at the Publicis Champs-Elysées cinema with a press conference at the Louvre Museum, which was attended by Toshiaki Karasawa (Kenji) and Takako Tokiwa (Yukiji).

[76][77] Manga critic Jason Thompson called 20th Century Boys "an epic saga of nostalgia, middle age, rock n' roll, and a struggle against an evil conspiracy."

Thompson wrote that despite being a manga aimed at a male audience, the series gained fans of all ages for its great premise, storytelling and the mystery behind Friend.

[78] In addition to King's It, Thompson and Tom Speelman of Polygon both suggested that the Aum Shinrikyo doomsday cult also served as an inspiration on the manga.

[78][79] Anime News Network's Carlo Santos felt the pacing of the series is very well written, and praised the intricate and interconnecting plot and its twists, as well as the well-developed characters.

[80][81][82] He also noted Urasawa's art and dialogue, saying "it takes real skill to build a story as multi-layered as this one and still have it make sense as the characters explain things".

"20th Century Boys becomes an experience featuring horror, science fiction, post-apocalyptic futures, wild humor, epic landscapes, and more as an apex accomplishment in manga.

He praised the action and faithfulness to the original manga, but stated that those unfamiliar with the source material may find the large cast of characters and complex story confusing.

[87] A fan of the manga, Jamie S. Rich of DVD Talk felt too much had to be cut to fit three films, with the development of characters suffering as a result.

[88] In an opposite view, both The Guardian's Cath Clarke and Time Out London's Trevor Johnston gave the first film two out of five stars and cited the faithfulness to the manga as a negative, feeling that some of the material could have been cut.

However, he praised the character Friend and Etsushi Toyokawa's performance as Occho, as well as the ending that makes the viewer anticipate the final installment in the trilogy.

[92] On the third film, Burl Burlingame of the Honolulu Star-Bulletin wrote, "The steam seems to have run out of the franchise during this third part, and it's simply an OK capper to the series", but praised the special effects.