Part of the Alliance of Four Great Races, the Asgard were among the most advanced races in the universe, rivaled only by the Ancients, the Nox, and possibly the Furlings. They assumed the role of the Ancients as protectors of the Milky Way when the latter passed from existence and in turn passed that role down to the Tau'ri, to whom they gifted all of their knowledge and latest technology.
Over 30,000 years ago, the Asgard were a tall (approximately 2m), generally humanoid species that reproduced sexually. Over time, their physical form changed. The "modern" Asgard averaged about one meter in height, with grayish skin tones, small, skinny limbs, large heads and black eyes (bearing a striking resemblance to the "Roswell Grays" described in countless UFO stories). Their body, now weakened by several millennia of cloning is more than susceptible to the discharge of a zat'nik'tel (Loki fell almost immediately while most human and Jaffa stand for a moment).
The species possessed no adrenaline glands within their bodies and did not have epinephrine pumping through their lymphatic system.
The Asgard, pursuing means of extending their lifespans, began to use cloning technology. The mental patterns of Asgard that became ill or fatally injured were preserved by "downloading" them into computer memory crystals. The patterns were later placed in a new cloned body. This made the Asgard effectively immortal, but they lost their ability to reproduce sexually. They also lost the ability to evolve as the Ancients did, making Ascension impossible without assistance.
The excessive use of the cloning process began to damage and degrade the Asgard genome. This would result in the eventual extinction of the race unless a cure could be found. The Asgard devoted considerable time and resources to the project, but their best efforts proved fruitless. (The Vanir had more luck in dealing with this issue than their Ida counterparts, primarily because they were willing to experiment on humans when the main faction would not. Even so, they were not able to eradicate the problem, only postpone it.) The final attempt to solve the problem resulted in a rapidly progressing, fatal disease.
The Asgard evolved on the planet Othala, in the Ida galaxy. Their collected history and culture would span 100,000 years. They began exploring outside their galaxy around 28,000 BC. At some point, the Asgard began a program of cloning their people and transferring their minds into these bodies as a means of vastly extending their lifespans.
30,000 Years Ago
About this time, an Asgard ship (its crew placed in suspended animation) left Othala. It's navigational computers were damaged at some point, and the ship drifted across galaxies, ultimately ending up in the Milky Way (whether or not the Milky Way was its intended destination is unclear).
10,000 Years Ago
The Asgard made peaceful contact with the Ancient, the Furlings, and the Nox. They also came into conflict with the Goa'uld. Liberating several groups of Humans from Goa'uld worlds, the Asgard declared themselves their protectors until their cultures had sufficiently evolved, and threatened the Goa'uld with violent retaliation if they attempted to re-enslave the Humans.
The Vanir, a group of Asgard who did not share their brethren's ethical concerns about experimenting on humans to solve their genetic degradation problems, left the Ida galaxy and traveled to the Pegasus galaxy where they could continue their experiments, which were able to slow the advance of, but not reverse their genetic problems.
After the Ancients discovered they could not win their war with the Wraith and left for the Milky Way, the Wraith attacked the Asgard. The Asgard's intergalactic ships were damaged beyond repair in the first battle, and, as they lacked the means to construct new ones, they were stranded in the Pegasus galaxy.
In order to survive the Vanir moved to a Toxic planet the Wraith could not endure. At first they only needed to use simple breathing apparatus to survive, but the planet's atmosphere continued to degrade, forcing them to develop Armored exoskeletons capable of withstanding the new conditions. (Eventually, conditions would became too harsh, and they would be forced to venture out into the galaxy again).
Past to the Current Era
Despite the efforts of the Asgard, the Goa'uld became a great threat to the Milky Way. The Goa'uld began taking humans as hosts during this time, which greatly angered the Asgard, who realized humanity had a great potential (much like its parent race, the Alterans, later known as the Ancients). The Asgard's vastly superior technology was more than a match for the Goa'uld, who were forced to sign a treaty with the Asgard in order to keep from being destroyed.
Unwilling to completely give the galaxy over to the Goa'uld, the Asgard freed and transplanted humans on various worlds and posed as their protective gods as they felt they were not yet ready to have their belief system completely stripped away (This gave rise to Norse mythology on Earth). Accordingly, they devised a series of challenges designed to determine when these people were advanced enough to learn (and presumably cope with) the truth about their protector's true nature.
To ensure the safety of many of these planets, the Asgard and the Goa'uld brokered the Protected Planets Treaty. The treaty stipulated that the Asgard would allow the Goa'uld various benefits (the precise terms are unknown), and in return the Goa'uld would leave the humans of the various worlds mentionned in the treaty alone. The treaty could be expanded to include other worlds if both parts agreed to negotiate.
For some time the Asgard protected these planets with their advanced technology and warships but the discovery and invasion of the Replicator in their home galaxy forced the Asgard to draw away resources they had been using to enforce the treaty, leaving the planets protected in name only.
A long lost Asgard ship was recently found by Asgard scientists. Within it was one perfectly preserved Asgard ancestor. The Asgard scientist Heimdall began research on it since the clone was from a time when their cloning program was "not yet irreversible".
The Asgard's energy-based weapons were ineffective against the Replicators. Human weapons, on the other hand, used simple chemical reactions to drive metallic projectiles through kinetic force, something the mechanoids could not defend against.
With the aid of SG-1, the Asgard imprisoned all the Replicators in a time dilation field. Eventually, however, they escaped and attacked the new Asgard homeworld Orilla. Aegir attacked the ship that dropped out of hyperspace before it could raise its shields. Not all the Replicators were destroyed, and the Replicator Fifth managed to land on the planet.
Colonel Jack O'Neill, with the knowledge gained from the Ancient' Repository of knowledge' invented a weapon that blocked the network of intercommunication between the Replicator cells permanently. Thor was able to construct a larger version capable of affecting an entire planet.
After the Replicators
Although the Goa'uld and the Replicators had been defeated, new threats arose, such as the Ori. They continued to provide aid to Earth by providing technologies for the development of the Daedalus-class, and "lending" them the Asgard engineer Hermiod. The Asgard also participated in such critical events as the Battle of P3Y-229.
Their final attempts to repair the genetic degradation caused by continuous re-cloning failed, and unwilling to allow their technology to be plundered by other races, the Asgard decided to commit mass suicide and destroy their planet, but not without passing on their legacy. They gave Stargate Command an Asgard computer database, and made significant modifications to the Odyssey consisting of all their most recent technology.
As the upgrades were completed, three Ori warships arrived and attacked both Orilla and the Odyssey. The Odyssey fled with one Ori ship in pursuit, while the other two prepared to attack the planet. At that moment, the Asgard caused Orilla to explode, destroying the planet, the two ships, and the Asgard. The Asgard's upgrades to the Odyssey saved the ship and gave the Tau'ri an advantage in the war as the new plasma beam weapons installed on the Odyssey gave the Tau'ri weapons capable of destroying Ori motherships and almost any other ship they came across.
A few months after the death of the Asgard, the Atlantis expedition encountered Fenrir, an Asgard scientist who had been exiled from his people after his test of a new weapon resulted in the destruction of an entire solar system. Although Fenrir was naturally devastated when he learned of the loss of his people, he nevertheless sided with the expedition over the Wraith after Teyla Emmagan assured him that they had only deceived him to spare him the pain. He was subsequently killed when a Wraith Queen sacrificed herself to mount a suicide bomb run on his ship, but Colonel Samantha Carter was able to take control of his ship and use it to destroy the Wraith Hive ship.
The Vanir were encountered by the Atlantis expedition in 2008. After the expedition's discovery of Janus' lab, they invaded Atlantis, removed a key component of the Attero device, and captured Dr. Rodney McKay and Dr. Daniel Jackson. They forced Jackson and McKay to activate the device as a means to render the Wraith unable to use their hyperspace drives. The Vanir didn't care that a side effect of the device would cause any activated Stargate to overload and explode.
Their plan was foiled when Lt. Colonel John Sheppard took a Traveler ship to the planet and destroyed two of three Vanir spaceships in battle while the third escaped to hyperspace. The device was shut down by Jackson and McKay and subsequently destroyed by a Traveler ship from orbit.
The Asgard High Council was the government of the Asgard. The size and structure of the council is not known, but there were seven seated members, including Freyr, Thor, and Penegal. The High Council made decisions that affected the entire race. In addition, they negotiated in name of the Tau'ri in the Protected Planets Treaty. The council members met in a great room with holographic projectors.
Interaction with other races
As a function of the Protected Planets Treaty, the Asgard set up protective measures and holographic representations of themselves on a number of planets in the Milky Way galaxy for protection against the Goa'uld and other enemies.
On the planet Cimmeria, the Asgard Thor set up Thor's Hammer, a protective obelisk that transported Goa'uld or symbiote-carriers to an underground labyrinth where only the host can leave, the symbiote dying when passing through the 'hammer'. The 'hammer' was destroyed by the SG-1 when Teal'c was trapped in the labyrinth upon arriving at the planet. Thor replaced the destroyed hammer with a new one that would recognize Teal'c, and therefore, not harm him.
While the Asgard maintained a fleet of ships with defensive capabilities, they were not a militaristic race, and strongly disapproved of the use of technology for aggressive reasons. While they formed an alliance with the Tau'ri and were willing to give them upgrades for their ships such as Asgard shields, Asgard sensors, Asgard transporters and Asgard hyperdrives, they were unwilling to give up Asgard weapons until their race was on the brink of extinction due to a policy of not giving other races weapons, partially out of fear they'd be used by those other races against them. They also have safeguards built into the Asgard transporters to prevent it being used as a weapons delivery system.
It is generally theorized that the Asgard language influenced the evolution of Germanic languages. The written language of the Asgard is composed of runes (seen on panels of Thor's ship). The known named Asgard all had names that came from Nordic mythology.
Dependent on Neutronium, the Asgard possessed a level of technology vastly superior to that of Goa'uld and many other races throughout the universe. The only races with a comparable level of technology were the Ancients, the Ori, and possibly the Nox and Furling.
Their ships were capable of traversing thousands of light years in minutes, and their shields could withstand multiple hits from Ori warships while remaining undamaged. They had also mastered advanced transportation and holographic technology.
Shortly before their extinction, the Asgard developed energy weapons capable, with repeated strikes, of defeating Ori shields, which had previously proved impervious to everything except the unstable vortex of the Ori's own Supergate.
The sum total of the Asgard's knowledge and technology now belongs to the Tau'ri; a gift to them before their destruction. However, it will likely take Earth decades, maybe even centuries, to fully understand and put it to widespread use. However, the Tau'ri have had at least some success with this as they have been able to replicate the plasma beam weapons the Asgard gifted to them and put them on all their ships within a matter of months of first receiving the technology.
Little is known of Asgard military tactics as their advanced technology tended to dominate their enemies. They used this to achieve victory.
The Asgard were not above employing trickery against their enemies, such as bluffing the Goa'uld with the Protected Planets Treaty by appearing to be capable of destroying the Goa'uld while actually lacking the resources to intervene directly due to other problems. The Asgard had no hesitation in sacrificing themselves in battle, as was the case when Thor decided to self-destruct his ship in hyperspace in order to destroy a Replicator cruiser heading towards his new homeworld of Orilla.
The Asgard had some problems with developing creative or original military tactics, as their brains were simply too advanced and complicated, making them almost incapable of resorting to more primitive means in defeating their enemies. This was the primary reason why the Asgard took Major Samantha Carter to develop a tactic to use against the Replicators despite the fact that she was one of the smartest people on Earth.
Protected Planets Treaty
On one occasion the Asgard came to help Earth when the Goa'uld decided to attack. The Asgard met with representatives of the Goa'uld, and humans, in order to put Earth under the protection of the Protected Planets Treaty. This treaty had served to maintain peace between the Goa'uld and the Asgard. Attending in name of the Goa'uld were Yu, Nirrti and Cronus. Thor represented the Asgard and chose Colonel Jack O'Neill to represent the humans.