JLA/Avengers (Issues #2 and #4 are titled Avengers/JLA) is a 2003-2004 published by and , and features the two companies' teams of , DC Comics' and Marvel's .
In 1979, DC and Marvel agreed to co-publish a crossover series involving the two teams, to be written by and drawn by . The plot of the original crossover was a time-travel story involving Marvel's and DC's . Writer/editor was supposed to script the book, based on Conway's plot, and although work was begun on the series in 1981 (and Pérez had pencilled 21 pages by mid-1983), editorial disputes prevented the story from being completed. The failure of the JLA/Avengers book also caused the cancellation of a planned sequel to the 1982 crossover , .
An agreement was eventually made between the two companies in 2002, with the story to be written by and drawn by George Pérez. In a joint panel at 2000, Busiek, Avengers writer at the time, and , then-writer of the JLA, stated they had nearly come to an agreement to begin the crossover within the regular issues of their respective books but the two companies could not come to a business arrangement. Later, when the book became a "go project," Waid had just signed an exclusive deal with , so the writing chores were offered solely to Busiek.
Avengers/JLA eventually became a four-issue jointly published by and in late through early . The mini-series was published in the 48-page Prestige Format. Issues #1 and #3 were published by Marvel Comics under the title JLA/Avengers, while issues #2 and #4 were published by DC Comics with the title Avengers/JLA. The entire story was reprinted by DC Comics in 2005 as a two-volume collector's edition hardcover, and then re-released as a trade paperback in November 2008.
, an exiled immortal from the who gained the powers of , begins destroying entire universes in his obsession to learn what happened before the . The , an alien of the who is obsessed with games, offers to tell Krona about someone who has the knowledge he seeks; but only if Krona can beat him in a game. The game involves manipulating the Avengers and the Justice League into trying to obtain twelve items of power (six from each universe - the DC items being the ; the ; the ; the 's Medusa Mask; the Bell, Jar and Wheel of the ; and the of ; and the Marvel items being the ; the ; the ; the ; the ; and the ) that have been hidden around across the two worlds. The New God of the DC Universe also helps to trick the heroes into competing.
Numerous battles ensue, comparing and contrasting the powers and personalities of each universe's heroes. The opposing teams continue to skirmish until all of the items are gathered, at which point Krona and the Grandmaster betray each other. Krona lethally blasts the Grandmaster and scans his mind to learn of , while the Grandmaster uses the power of the twelve artifacts to trap Krona between the two universes.
Krona breaks free and begins to merge these incompatible universes, which would result in their destruction and the birth of another universe. The Avengers and the Justice League join forces to battle Krona, with a rapidly varying roster of characters and forms of characters drawn from the entire histories of both teams. Krona is finally defeated, and implodes to become a "". The two universes are returned to normal with no evidence of Krona's tampering, and the cosmic egg is later revealed to be monitored by the JLA.