The Marvel Cinematic Universe (MCU) is an American media franchise and shared universe centered on superhero films and television series starring various titular superheroes independently produced by Marvel Studios: based on characters that appear in American comic books published by Marvel Comics. The shared universe, much like the original Marvel Universe in comic books, was established by crossing over common plot elements, settings, cast, and characters. Due to the galaxy-spanning nature of the franchise, multiple different species have been introduced.

The Asgardians (based on the Marvel Comics race of the same name), or the Æsir - introduced in Thor, are the inhabitants of Asgard, on whom the gods of Norse mythology are based in MCU mythology. In the first film, they are shown as a race of superhuman extraterrestrials who look similar to humans but possess a highly advanced form of technology resembling magic and sorcery, upon which their entire civilization is built. However, later films contradict this, with Thor: Love and Thunder establishing that Thor and Zeus are actual gods in the fictional universe. The film's post-credit scene also confirms the existence of Valhalla in the Marvel universe.

The Asgardians are shown suffering major losses, first during Hela's attempt to harness Asgard's power in Thor: Ragnarok, and later by Thanos on his quest to retrieve the Tesseract, as revealed in Avengers: Infinity War. A group of survivors escape to Earth, and build a settlement under the rule of King Valkyrie.

They appear in the following films - Thor, The Avengers, Thor: The Dark World, Avengers: Age of Ultron, Doctor Strange, Thor: Ragnarok, Avengers: Infinity War, Avengers: Endgame and Thor: Love and Thunder; as well as the ABC series Agents of S.H.I.E.L.D., the Disney+ series Loki and the animated series What If...?.

Celestials (based on the Marvel Comics race of the same name) are an ancient race of entities introduced in Guardians of the Galaxy and later expanded upon on Guardians of the Galaxy Vol. 2 & Eternals as beings with vast matter & energy manipulation abilities.

In Guardians of the Galaxy, they are introduced as being present in the universe long before the galactic communities and even the Asgardians. Their origin and nature are not elaborated upon. Whatever is known about them is known only by a few characters, such as Taneleer Tivan, who reveals that the Celestials utilized the Infinity Stones as a means of power against lesser life forms. The severed head of a deceased Celestial, which was converted into Knowhere, appears in these films: Guardians of the Galaxy, Avengers: Infinity War, The Guardians of the Galaxy Holiday Special and Guardians of the Galaxy Vol. 3.

Eson the Searcher is shown in a flashback sequence as a former owner of the Power Stone in Guardians of the Galaxy. He uses its power to level the surface of an entire planet.

In Guardians of the Galaxy Vol. 2, Ego the Living Planet is a Celestial who controls a humanoid avatar to travel the universe. His planetary form is a living extension of his Celestial consciousness. Over the course of many years, he plants thousands of alien-seedlings to expand his existence across all life sustaining worlds. However, Ego needs another Celestial's assistance to activate them, so he fathers children with various alien races and had Yondu Udonta retrieve them which meant he could gauge their Celestial powers. Peter Quill is the only child who gains his father's Celestial abilities, though he loses them after killing Ego and foiling his plans.