The Cylons are a cybernetic civilization at war with humanity in the science fiction movie and television series Battlestar Galactica, in both the original 1978/1980 series and movie and the new "reimagining" of 2003/2004. Their nature and origins differ greatly between the two Galacticas.

Battlestar Galactica (1978) & Galactica 1980

Cylon models

In the 1978 Galactica series and the 1980 spin-off, the Cylons were created to serve the reptilian race of the Cylon Alliance. Somehow, just prior to the start of The Thousand Yahren War, the android Cylons made a deal with the mysterious Count Iblis* and destroyed their biological masters. They soon devoted themselves to the genocide of all other humanoid races in the galaxy. *(Means "Satan" in the Arabic language)

Centurions are armed with a powerful energy weapon, often referred to as a blaster rifle. They also have bayonets and swords for close combat and execution of prisoners.

Cylon society is composed of five models, four of which have been well-documented:

• Imperious Leader: The leader of the Cylon Alliance and highest Cylon model. The Imperious Leader is an IL-Series Cylon with some extra augmentation, including a third brain and a body shell resembling the reptilian Cylons. One was killed at the Battle of Carillon and another was likely killed during the Battle of Gamoray . The voice of the Imperious Leader is identical to that of Count Iblis.
• IL-Series: Acts as a commander for the military and governor for civilians of the Alliance. They have two brains, a humanoid face and wear shimmering cloaks. They are almost feudal in nature and bicker among themselves frequently. This could be a tactic to help the most skilled IL-Series ascend the throne.
  • Lucifer - Baltar's second in command. Presumably led pursuit of the Galactica after Baltar's capture.
  • Spectre - Commander on the planet Atilla in The Young Lords. Enemy of Lucifer.
• Command Centurion: Centurions with gold armor. These are the lower commanders for individual military units. At least one, Vulpa of Gun on Ice Planet Zero fame, has a name.
• Centurion - Centurions with silver armor. Basic centurions make up the ground forces and pilots of the Alliance military.
• Android: A Cylon android was featured on the Halloween episode of Galactica 1980. This model had a superficial human appearance, a condescending/superior attitude toward Centurions, and was believed to be entirely electromechanical.

There is also a unique Cylon with glittering robes, but what looks like a mask attached to its face. They are seen in the Imperious Leader's delegation to Gamoray in The Living Legend.

The Cylon Centurions – the type most often depicted in the original Battlestar Galactica – were strikingly similar to the Imperial Stormtroopers of Star Wars (and the Imperious Leader was an obvious parallel to the evil Emperor). The similarities were noted by many at the time and may have been one of the factors that prompted 20th Century Fox's lawsuit for plagiarism against Universal Studios, the owners of Battlestar Galactica.

Cylon spacecraft

• Cylon Raider: A heavy fighter with a crew of 3, two pilots and a commander. They are armed with dual-firing weapons.
• Basestar: A warship more than a match for a Colonial battlestar, a basestar mounts two pulsars, many laser turrets, and carries 300 Raiders.

Battlestar Galactica (2003/2004)

In the 2003/2004 miniseries and full-length series which "reimagined" Battlestar Galactica, the Cylons are fundamentally different. They were created by humans as robotic workers and soldiers; executive producer Ronald D. Moore has hinted that they were used in wars between the human Colonies. Eventually they gained sentience, rebelled and fought the humans in a major conflict that devastated both sides. An armistice was agreed, following which the Cylons disappeared for over forty years. They returned en masse, launching a sneak attack with nuclear weapons which eliminated most of the colonies (similar to the original Galactica). This is the starting point of the 2003 Battlestar Galactica miniseries.

Cylon models

The miniseries establishes that there are only twelve models of Cylon. It is not yet known exactly how this is divided up – for instance, whether the new humanoid Cylons constitute a single model with variants, or whether each humanoid variant is a model in its own right. There is also now no distinction between Cylon spacecraft and the Cylons, as the spacecraft are Cylons.

The fact that there are twelve Cylon models, to match the twelve Colonies and the twelve human gods, is supposed to have some hidden significance – though executive producer Ronald D. Moore has not yet said what it is!

• 'Original' Centurion: An "original" set of Cylon Centurion armour is seen very briefly in the miniseries, preserved in the Battlestar Galactica museum, just before the outbreak of war. One of the new humanoid Cylons comments that they are still around, as they "have their uses", suggesting that the Centurions (and perhaps also the Raiders) may be lower orders in the Cylon heirarchy.

• Centurion: The new "Centurion" models have an updated look, retaining the silver appearance, robotic body, helmet-like head and red bar eye. They are larger, stronger, and more agile than the originals. They also have retractable weapons in their lower arms. Unlike the original Cylons, they can definitely shoot straight.

• Raider: The new Raider is a biomechanical fighter craft with twin cannon weapons and FTL (Faster-than-light) capability. Note that unlike the original Raiders, these do not carry passengers or crew; the ship itself is a Cylon. It appears to be an oxygen-breathing biomechanical organism housed within a metallic exoskeleton. In one episode of the 2004 series, the stranded Starbuck finds a downed raider and disovers that once inside its "head" she can control the propulsion and weapons systems by manipulating its biomechanical actuators. Another (Cylon) character describes the Raider as being somewhat like a pet, implying a more specialised order of intelligence, perhaps comparable to that of a dog or another large predator such as a big cat. Intruigingly, the captured Raider's internal 'organs' do not appear to decompose, even though its brain has been "killed" by a shot from Starbuck's Viper, .

• Basestar: As before, these immense craft act as carriers for squadrons of Raiders. They are themselves biomechanical entities, with fleshy "hangars" for Raiders to occupy. It is unclear whether the Basestars are one of the Cylon models or not.

• Infiltrators: The biggest change of all: Cylons that look exactly like humans. Created using advanced biological technologies, these specialized models are used for infiltration. The revelation that Cylons can perfectly mimic human form, even down to the internal organs and lymphatic system, creates a wave of fear and paranoia among the humans. Some Infiltrator-model Cylons are "sleeper" agents, and like the humanoids in Ridley Scott's Blade Runner, they have been implanted with false memories so they think they are a human, presumably until they are activated for a specific mission. They can interact with humans very convincingly, even being capable of intimate emotional and sexual relationships, although spines of female stealth Cylons pulse with a red glow during intercourse. The reason for this is not revealed, nor has it been revealed if male stealth Cylons react similarly. The first season finale also suggests that Human/Cylon hybrids are also possible. So far, four humanoid Cylons have been depicted:

• • "Aaron Doral" – three copies are shown. The first is as a public relations worker for the abortive Battlestar Galactica museum, who is revelead to be a Cylon in the opening miniseries and is stranded at Ragnar Anchorage by Adama. The second is suicide bomber who narrowly misses killing Adama and Tigh. The third is one of the 'controllers' who monitors the progress of Helo and the duplicate Boomer on Cylon-occupied Caprica, working alongside one of the duplicate Number Sixes. [1]

• • Lieutenant Junior Grade Sharon "Boomer" Valerii – two versions, a sleeper agent on Galactica who does not know that she is a Cylon (although she suspects it), and a fully aware version who accompanies a fellow Galactica crewmember (who does not know that she is a Cylon) who has been stranded on Cylon-occupied Caprica. [2]. The revelation she is a Cylon agent implies that Helo's stranding and Baltar's apparently fortuitous rescue, may be only part of an elaborate Cylon plan.

• • "Leoben Conoy" – two copies are shown in the first series. The first is a smuggler who is discovered in the munitions depot at Ragnar Anchorage and is killed by Adama, who has the body taken back to the Galactica. The second is an agent provocateur, an apparently unsuccessful saboteur and who is captured aboard a passenger ship, claiming to have planted a nuclear warhead somwhere in the fleet. He is tortured by Starbuck but just before he is executed on the President's order, he tells tells her that Adama is a Cylon.

• • Number 6 – is a beautiful female model evidently designed for seduction. (The name may be a reference to The_Prisoner). In the first episode she is shown having an affair with Baltar (and possibly others) on Caprica, and she was the Cylon "Trojan horse" who disabled the Colonial defence system mainframe, allowing the Cylons to annihilate the Colonies. One of the series' most effective plot devices is that the Number Six on Galactica can only be seen by Baltar -- in every episode she appears to torment Baltar in disturbing waking visions, as well as making him 'relive' his last hours with her on Caprica. It is not clear whether she is a product of his incipient madness, as he first thinks, or if (as seems more likely) she is a projection of a chip that she implanted in his brain. She does appears once on Galactica in physical form, calling herself as "Shelly Godfrey", but whether Shelley is the physical manifestation of Baltar's 'dream girl' (she coincidentally diappears from his visions for the duration of her appearance on board Galactica) or whether there is another Number Six hiding among the fleet is not made clear, although after her discovery and abrupt disappearance, Adama finds the glasses she was wearing, suggesting that she is indeed real. There are also several copies of Number Six on Caprica including the copy working with the third Doral copy and observing Helo and Boomer. The Cylon religion appears to be of central importance to her, she refers to "God" in almost every episode, and part of her mission is evidently to convert Baltar to the religion of the Cylons' "one true god".

Apparent weaknesses in the Infiltrators include heightened susceptibility to radiation and a perhaps too-close duplication of human frailties, such as fear, jealousy, pride, lust, wrath and zealotry. Their superior strength and endurance is an additional, although likely necessary, deviation from human norms which could "give away" a stealth unit. Boomer's copies shows suspicious endurance on Galactica and Caprica, which is noticed by their human counterparts. The humanoid Cylons are not merely biological clones of humans: they have one major chemical difference, posessing "silica pathways" or "silica relays"— (presumably in their nervous system) and both terms are used in the miniseries—in the body or brain. Exactly what this does is unclear, though it is presumably the remnant mechanical aspect of the Cylons' biomechanical technology.

Unlike many sci-fi robotic races, including the original Cylons, these re-envisioned Cylons are not ignorant or naive about human emotions, and some even seem to have a mastery of the subject beyond that of the average human. One Cylon on Caprica is heard expressing doubt and apparent regret about the extermination of the humans.

A particularly compelling example of the Cylons' complex emotions is shown in the opening scene of the miniseries, when Number Six stops to admire a human baby in a pram, then kills it by breaking its neck. While the act at first seems callous and cold-blooded, it can also be argued that the comment she makes as she kills the child hints that she in fact thinks that she is mercifully sparing the child from further suffering in the impending attack. Conducting psychological experiments and playing mind games with their human enemies appears to be a common tactic, and they often are seen manipulating the emotions of humans in order to destroy or control them, most notably Baltar. Some fans of the original series refer to these new humanoid model Cylons as Biocylons, to distinguish between them and the original Cylons.

When an infiltrator's body dies, its consciousness is supposedly transmitted to another empty shell of the same model. The process is not perfect, however; the radiation in Ragnar Anchorage stops the first "Leoben Conoy"'s consciousness from transferring, and the second saboteur copy of "Conoy" claims that his actions have been prompted by a fear that the transmission process has a finite range and that he is too far away. Another curious aspect of Cylon behaviour when they encounter humans, they ask "Are you alive?", although the significance of this question is not explained.

Another fascinating divergence from the old series is that the Cylons now have a monotheistic religion of their own devising, in contrast to the polytheistic beliefs of the human colonists, (who worship a pantheon of gods including Artemis and Aphrodite) and the Cylons apparently believe that by destroying humanity they are fulfilling the wishes of God. Some have suggested that the Cylons are seeking to exterminate humanity to purge its wickedness, much like the Great Flood of Bible mythology, and that the Cylons consider themselves to be the "chosen people".

However other fans believe that the Cylons actually have a greater plan for mankind and allowed the Galactica fleet to escape the destruction of the Colonies on purpose in order to facilitate this 'plan.' This plan may include following the fleet to Earth to destroy the 13th tribe of humanity. The first season finale also suggests that a union of the Human and Humanoid Cylon species is a goal, although the extended experiment involving Helo and Boomer suggest that the Cylons want or need to know more about humans, and that a simple biological union is not their sole goal, since they could clearly kill Helo at any time and presumably take the requisite DNA from his body. Other fans believe the Cylons are toying with the mouse before they kill it; not so much for pleasure, but to learn more about the beings on which they are modeled and how they react to adversity.

Trivia

Sci-fi fans traveling through Hartsfield International Airport in Atlanta were always quick to note that the automated voice on the trains connecting the passenger concourses to the main terminal was a perfect match for the voice of a Cylon. Alas, in preparation for the 1996 Summer Olympics, the airport was spruced up and the Cylon voice was retired in favor of a pre-recorded "human" voice. The voice heard going down the escalator to the trains, however, still reminds them of the robot guarding the path to sanctuary in Logan's Run.

In the source code of Mozilla (and its Netscape predecessor), the "undetermined" progress bar that slides back and forth -- rather than filling up the bar from left to right -- is referred to as the "Cylon".