The Earth is not the only planet in the universe populated by sentient life. The universe is teeming with other life forms, the number of which is incalculable. Because the denizens of Earth have not yet perfected a practical faster-than-light stardrive enabling interstellar travel, the majority of the alien races encountered by Earth people are those races who have mastered interstellar travel themselves and have visited Earth. Due to the fact that the Earth's solar system is near a natural warp-space access point, there has been a high incidence of alien visitation since before human life first appeared. Indeed, numerous advanced civilizations left outposts on Earth during prehistory, including the Kree, the Lumina, the Arcturans and the Celestials. In the 20th Century, alien visitation reached epidemic proportions with well over eighty different alien races dispatching exploratory parties and, in some cases, armadas to Earth. The Earth has been of particular interest to the warring Kree and Skrull Empires, since the Earth would be a strategic military outpost from which to launch attacks on the other. One of the main factors accounting for the increase in the extraterrestrial investigation of Earth is the explosive growth of Earth beings who through mutation or transformation have demonstrated superhuman capacities. No other races of beings have manifested such a vast variety of powers as Earth people, and many races are curious to determine why. (The human race's capacity for benevolent mutation is a legacy of the alien Celestials who examined the genetic material of human beings at the dawn of man.) It is these superhuman beings who have been so phenomenally successful thwarting the selfish designs of the scores of alien invaders. Virtually every superhuman champion on Earth has had at least one extraterrestrial encounter.

The majority of the alien races documented here have humanoid configurations (bilaterally symmetrical, upright posture, two arms, two legs, brain located in head). Some are astonishingly similar to the human race, being virtually indistinguishable until the cellular level. Other races resemble humanoid versions of other Earthly animals, such as lizards, skunks, insects, fish, and so on. The reason for this staggering similarity in phenotypes among races evolving on countless worlds separated by millions of light years is not known. A being called "Xorr the God-Jewel" once claimed that the "seeds" of all humanoid faces were contained within Xorri, the now-extinct race that formed the jewel. The validity of this claim is questionable, but it is also unlikely that the vast similarities among certain races can be attributed to parallel evolution. There must something to account for so many coincidences among these alien races, but as yet there is no indication what it is. It is possible that humanoid races do not dominate the universe as it would appear, and that it is mere happenstance that the majority of the alien races Earth has encountered are humanoid.

Most of the alien races which comprise the Shi'ar Empire have yet to be identified, and so are not listed here. Also any races that have become extinct or one or two survivors with no means to perpetuate their species are listed here.