The Celestials debuted in Eternals #1 (July 1976) and reappeared as regular guest stars in three subsequent limited series sequels: Eternals vol. 2, #1 - 12 (Oct. 1985 - Sep. 1986), Eternals vol. 3, #1 - 7 (Aug. 2006 - Feb. 2007), and Eternals vol. 4, #1 - 9 (Aug. 2008 - May 2009).
The characters have also been featured in other titles, including the "Celestial Saga" storyline in Thor Annual #7 (1978), Thor #283 – 300 (May 1979 - Oct. 1980), Thor #387 – 389 (Jan. – March 1988), Quasar #24 (July 1991), Fantastic Four #400 (May 1995), X-Factor #43-46 (Aug. - Nov. 1989 and #48-50 (Dec. 1989 x 2 - Jan. 1990).
Celestials existed at the birth of the universe and created life and death along with the multiverse. Early on in the history of the universe, the hegemony of the Celestials was challenged by a species known only now as the 'Aspirants'; after a bitter war of attrition, during which a hyperweapon called 'Godkiller' destroyed countless Celestials, the 'Aspirants' were ultimately defeated.  New Celestials may be born from consuming the mass of an entire galaxy. Visiting Earth every few thousand years in groups called "Hosts", the group is revealed to have been responsible for the creation of the Eternals and the Deviants, and via genetic manipulation, the existence of superpowers in mutants. Resenting the presence of the Celestials and their monitoring of Earth's progress, the Skyfather figures of Earth (e.g. Odin, Zeus, and Vishnu) attempted to stop the Third Host, but were quickly outmatched. The Skyfathers then developed a convoluted plan to stop the Fourth Host via the use of the Odinsword and Destroyer armour, but once again the Celestials — although also opposed by Odin's son Thor — prevented the offensive and melted the Destroyer armour into slag, scattering the Asgardians' life forces. Thor threw the Odinsword through Arishem's chest, but he removed it, analyzed it and then vaporized it. The Earthmothers (such as Frigga and Hera) of Earth, however, made an offering of twelve perfect humans, which was accepted and the planet was spared judgment. The judgment process was eventually witnessed by Thor, who observed Celestial Arishem the Judge sending an execution code to Exitar the Exterminator, a 20,000-foot (6,100 m) tall Celestial who carried out Arishem's "sentence". Exitar terraforms the planet in question into a garden paradise, with only the "evil" inhabitants having been destroyed. On one occasion the hero, Quasar, observes a race completely fail the genetic test, with every living creature being destroyed with their planet. The Celestials' actions conflicted with the policy of "non-interference" practiced by fellow cosmic entities the Watchers, with the two races becoming enemies. The Celestials and their "opposites," a group of entities known as the Horde, are established as instruments of an entity referred to as the Fulcrum, their purpose to be "instruments of the planting/creation/teeming of the universe."
The team of space adventurers the Guardians of the Galaxy find and use as a base the severed head of a Celestial floating in an area of space known as "The Rip." Dubbing the structure "Knowhere," it also acts as a common port of call (complete with a market and bar) for travelers from all points in the space-time continuum. The base is administered by its chief of security, Cosmo, a telepathic and telekineticSoviet space dog originally lost in Earth orbit in the 1960s. Courtesy of the deceased Celestial's "Continuum Cortex", travelers with special "passport" bracelets can teleport to any point in the universe instantaneously.
It is implied that there was, at one point, literally billions of Celestials, until an alien race went to war against them by creating a now-lost hyperweapon called the Godkiller, a space-borne 25,000 feet-tall humanoid robot which dwarfs even the Celestials themselves, and was claimed to have destroyed most of the Celestials before they were able to stop it by wiping out the alien race that built it. 
Referred to as "space gods" by the Eternals and the Deviants, the Celestials appear as silent, armored humanoids with an average height of 2,000 feet (610 m). They are capable of feats such as reducing the Asgardian construct known as the Destroyer to slag, moving planets at will, and creating and containing entire pocket universes.Reed Richards theorized that the Celestials' source of power was Hyperspace itself – the source of all energy in the Marvel Universe. The characters are almost totally invulnerable, and have only been harmed in rare instances before instantly regenerating. The first known assassination of a Celestial was carried out by the Apocalypse Twins, who used the axe "Jarnbjorn", divinely enchanted to pierce Celestial armor, against the Celestial Gardener. The now-lost hyperweapon Godkiller, a space-borne humanoid robot which dwarfs even the Celestials themselves, was claimed to have destroyed Celestials literally by the billions. 
The characters also appear in the alternate universe limited series Earth X, appearing as beings of energy encased in armor composed of vibranium, a metal with properties that prevent their dissipation. They reproduce by planting a fragment of their essence in a planet, which matures into a new Celestial over the course of eons. The cosmic entity Galactus opposes them, devouring planets that incubate Celestial "eggs" to prevent the Celestials from overpopulating the universe. The Celestials also appeared in What If vol. 2, #1 (July 1989); What If? Newer Fantastic Four #1 (Feb. 2009) and What If: Secret Wars #1 (Jan. 2009).