Existing alongside the Marvel Universe are other realities, aspects of which sometimes affect events in the Marvel Universe. Some of these realties contain Earthlike worlds and operate under natural laws similar to our own; others are wildly dissimilar, and bear little resemblance to the reality we know.

Most (if not all) of the other realities which have thus far been encountered can be classified into one of several categories, including:

ALTERNATE EARTHS: These realities contain worlds that are very much like Earth, except that specific events did not occur in quite the same way. There are two types of alternate Earths - a Divergent Earth, in which the history of events did not differ from the history of our world until a particular event occurred, and a Parallel Earth, in which no particular point of divergence can be identified. It should be noted, however, that any Parallel Earth could, in fact, turn out to be a Divergent Earth upon further investigation.

ALIEN WORLDS: These realities contain worlds that are clearly not Earth, though they may be Earthlike in some respects. In most cases, these alien worlds will be inhabited by humanoid (if not human) beings, but intelligent life is certainly not a requisite for an alien world.

MICROVERSES: These realities are those, which can be reached through a process of "shrinking." When the space between atoms of physical matter is reduced to a certain point, that matter may exit this reality altogether. At one time, it was thought that microverse worlds existed within the atoms of our own universe, but it seems now that this is not the case. Rather, the access points between dimensions are microscopic in size, not the worlds themselves. Thus, it is possible to travel to the same "microverse" on more than one occasion even if the journeys begin at two different locations. (For more on the process of shrinking, see Ant-Man.)

ASTRAL DIMENSIONS: These realities do not contain physical matter, but ectoplasm, a quasi-material substance, which cannot be totally defined. It is difficult for physical beings to journey to Astral Dimensions. In most cases, entry to an Astral Dimension is restricted to a traveler's astral (or spirit) form, which is also composed of ectoplasm.

INTERDIMENSIONAL SPACE: These realities "between" realities tend not to have any recognizable stability to their structure. Some are featureless and have the appearance of unending space. Others contain bizarre phenomena, which follow no consistent set of natural laws that we can observe.

MYSTIC REALMS: These realities are governed by natural laws which lend themselves to an interpretation of "magic." While such worlds could, also be classified as "alien," that designation is usually reserved for worlds whose natural laws more clearly resemble Earth's.

ANTI-MATTER UNIVERSE: Only one reality is known to be composed of anti-matter. The distinction is worth drawing, since under normal conditions matter and anti-matter will annihilate each other on contact, but in all other respects the Negative Zone can be said to contain simply alien worlds.

The list below contains important information regarding the many alternate realities that have appeared in the Marvel Universe up to 1985. The list includes some entries which may not in fact be distinct realities of their own, but may be a part of other alternate realities.

The set of all realities described in this list, as well as others yet to be discovered is called the Marvel Multiverse.

Specifically excluded from this list of alternate dimensions are alternate time-lines such as those of Kang, Killraven, the Guardians of the Galaxy, and more recently The Age of Apocalypse. While various alternate futures contain their own distinct realities, we deal here only with worlds which are contemporary with the Marvel Universe and which do not require time travel to reach.