Within Marvel Comics, most tales take place within the fictional Marvel Universe, which in turn is part of a larger multiverse. Starting with issues of Captain Britain, the main continuity in which most Marvel storylines take place was designated Earth-616, and the Multiverse was established as being protected by Merlyn. Each universe has a Captain Britain designated to protect its version of the British Isles. These protectors are collectively known as the Captain Britain Corps. This numerical notation was continued in the series Excalibur and other titles. Each universe of the Multiverse in Marvel also appears to be defended by a Sorcerer Supreme at nearly all times, appointed by the mystic trinity of Vishanti to defend the world against threats primarily magical in nature from within and beyond and bearing the Eye of Agamotto.

This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these template messages) This article may be too technical for most readers to understand. (September 2010) This article may require cleanup to meet Wikipedia's quality standards. The specific problem is: Few sources, no real-world context. (April 2023)

Later on, many writers would use and reshape the Multiverse in titles such as Exiles, X-Men, and Ultimate Fantastic Four. New universes would also spin out of storylines involving time-traveling characters such as Rachel Summers, Cable, and Bishop, as their actions rendered their home times alternate timelines.

The Multiverse also plays a role in the Marvel Cinematic Universe (MCU), with the central universe having being known as Earth-199999 in external media and Earth-616 in internal media. Its concept was first introduced in Doctor Strange (2016) before becoming the focal point of the franchise in "The Multiverse Saga" (2021–present). Additionally, the Multiverse has also been explored in the X-Men film series, Sony's Spider-Man Universe (SSU), and the Spider-Verse franchise.

The Multiverse is the collection of alternate universes that share a universal hierarchy. A large variety of these universes were originated from another due to a major decision on the part of a character. Some can seem to be taking place in the past or future due to differences in how time passes in each universe. Often, new universes are born due to time traveling; another name for these new universes is an "alternate timeline". Earth-616 is the established main universe where the majority of Marvel books take place.

The Marvel multiverse is protected from imbalance by the Living Tribunal, a vastly powerful humanoid cosmic entity, who is one for the entire multiverse. It may act to prevent one universe from amassing more power than any of the others or from upsetting the cosmic balance in some way. It is only overseen by the One-Above-All, an omnipotent entity said to have created the entire Marvel Multiverse.

According to the origin mythology, at the beginning there was only one universe, The First Firmament, but due to actions of Celestials existing there, it diverged.^[1] Then, the Multiverse went through several incarnations and eventually the Big Bang caused the existence of the Seventh Cosmos, where most well-known heroes originated. The seventh iteration of the Multiverse was destroyed as a consequence of the phenomena known as incursions and was eventually reborn as the eighth thanks to Reed Richards.^[2] According to him the ultimate fate of the Multiverse is to perish in all-encompassing heat death.^[3]

According to Forge, mutants living on these alternate Earths have lost their powers due to M-Day, as stated in "Endangered Species"; however, this mass depowering has not been seen in any of Marvel's current alternate reality publications such as Exiles, the Ultimate Marvel titles, Amazing Spider-Girl, the Marvel Adventures titles or GeNext, though it is possible that the issue of time may be related to their exclusion. This was apparently retconned during the "X-Men: Messiah Complex" storyline, where Forge stated that all mutants in possible future timelines were depowered, not in parallel universes.^[4] This, in addition to A.R.M.O.R.'s observation that Lyra arrived from an alternate reality^[5] indicates that the topology of the Marvel Multiverse is based on new realities branching off from key nodes of a timeline instead of strictly parallel dimensions.

The classification system for alternate realities was devised, in part, by Mark Gruenwald.^[6]

As stated above, nearly every imprint, timeline and appearances in other media have its own separate universe. Most of these have been cataloged by Marvel Comics in many publications, being most notable the Official Handbook of the Marvel Universe: Alternate Universes. The numerical designations for these are rarely revealed outside of reference works such as the Official Handbook of the Marvel Universe: Alternate Universes 2005. A.R.M.O.R. and Project Pegasus however seem to possess vast knowledge of other Marvel realities, using the same designations; whether this is simply narrative convenience on behalf of Marvel's authors or an unusual decision by these agencies to use an effectively alien catalog method is as yet unstated.

The numeric designations of these alternate universes have been confirmed by Marvel Comics throughout the years and compiled in 2005's Official Handbook of the Marvel Universe: Alternate Universes, and in Marvel publications since the release of the Handbook.

Many official numbers are random or use other numbers as a base, the best example of this is Ultimate Marvel. 1610 is the swapped numbers of 616 with a 0 to differentiate it from the already existing 161. In addition, many universes have also been designated with numbers by fans with various methods for the numbering, such as the birth date of an important Marvel staff member (artist Nelson Ribeiro for the Transformers U.S. Universe, Earth-91274) or the spelling of a name with a touch-tone phone (Animated Silver Surfer Earth-936652, spells out Zenn-La).

In 2014, during the publication of the "Spider-Verse" storyline, writer Dan Slott posted on Twitter that the numbers that appear in wiki entries and handbooks do not count, only those that are published within "actual" stories do. This was in response to the questions that the different numbers for some Earths appearing in Spider-Verse brought up, such as the Spider-Friends being from Earth-1983 and not the believed designation of Earth-8107.^[9] This has created some debate among readers, as some believe that the "Spiders" with numbers that do not match the "original" ones are alternate versions, or if the former numbers should be completely dismissed, despite being official.

In the 2015 Secret Wars series, a confrontation with the Beyonders over the fate of the various alternate versions of the Molecule Man results in the destruction of the Multiverse, triggering various 'incursions' as Earths crash together and destroy each other, the Beyonders' assault culminating in Doctor Doom stealing the power of the remaining Beyonders and bringing the last of the parallel universes together into a single 'Battleworld'. Doom rules this reality for eight years until key heroes and villains from the pre-existing Multiverse are discovered and released by Doctor Strange, who had been acting as Doom's 'sheriff' until the discovery of the survivors gave him an alternative. In the heroes' final assault on Doom's fortress, the Molecule Man, who had been the source of Doom's power, transfers Doom's power to Mister Fantastic when Doom acknowledges that Reed would have done a better job as 'God' than he did. Having restored Earth-616 as it was before the Beyonders' incursions began, Mister Fantastic departs to recreate the multiverse with the company of his restored family.

• Marvel Universe
• Multiverse (DC Comics)