A cloaking device is a form of stealth technology that uses selective bending of light (and other forms of energy) to render a starship or other object completely invisible to the electromagnetic spectrum and most sensors. It has been encountered in varying forms over the centuries.
Early cloaking technology
One of the earliest encounters with cloaking technology was in April 2151, when it was utilized by some agents of the Suliban Cabal to covertly approach the Enterprise and thus kidnap Klaang the Klingon from the ship's sickbay. The Suliban used cloaking devices on many of their ships, including the cell ships and the stealth-cruiser. It is likely that the use of cloaks by the Suliban was temporally anachronistic, and the technology had been given to them by their mysterious benefactor from the 29th century. (ENT: "Broken Bow")
During a battle near a Suliban helix in a class seven gas giant, Enterprise managed to capture a cell ship. Chief engineer Charles Tucker III studied the device in his spare time, and in 2152 the ship was used to mount a rescue mission. Unfortunately, Tucker's experience with cloaking technology was somewhat limited and he accidentally dosed his hand with the particles the ship used to generate the cloaking effect, temporarily rendering it invisible. (ENT: "Broken Bow", "The Communicator")
- The Suliban cloaking devices seem to use a form of particle radiation to alter the molecular structure of matter in order to allow light and sensor signals to pass through it. This explains why Mr. Tucker's hand was cloaked without any external power source or gravity field surrounding it. It would also explain why the quantum beacons could detect such cloaked vessels.
Enterprise learned how to penetrate a Suliban cloaking device in March 2152, when the temporal agent Daniels explained how to construct a quantum beacon so that Enterprise could reveal the location of the cloaked stealth-cruiser responsible for the destruction of Paraagan II. (ENT: "Shockwave")Enterprise was also responsbile for an encounter with another organization that used stealth technology, the Romulan Star Empire. In April 2152, the ship blundered into an entire cloaked minefield protecting a Romulan-claimed planet. The quantum beacons proved effective against the minefield, but when two birds-of-prey suddenly arrived, Enterprise was not able to penetrate their more advanced cloaks. (ENT: "Minefield")
The return of the cloak
By the 23rd century, the very idea of practical invisibility was considered only theoretically possible, due to the enormous amount of power required. This was proven untrue in 2266, when the USS Enterprise (NCC-1701) encountered a Romulan bird-of-prey that used the device to cross the Romulan Neutral Zone and wipe out several Earth Outpost Stations. The Enterprise was able to defeat the bird-of-prey in battle, partially because the cloaked ship was still visible to tracking sensors, but there was no doubt the device itself had been a success. (TOS: "Balance of Terror")
The cloaking device soon spread to the Klingon Empire as well. One of the first Klingon vessels to be equipped with the device was the IKS Klothos, whose Commander, Kor, used it to great effect at the Battle of Caleb IV. The device was soon used aboard many other Klingon vessels, such as the Bird-of-Prey. (Star Trek III: The Search for Spock; DS9: "Once More Unto the Breach")
- It has been speculated that the Klingon Empire acquired cloaking technology during an alliance with the Romulans. Such an alliance was loosely implied by Romulan use of Klingon ships in TOS: "The Enterprise Incident" and later mentioned as having taken place at some point in the past in TNG: "Reunion" and TNG: "The Neutral Zone" but there is no canon evidence to support the idea of Romulans supplying Klingons with cloaking technology.
The Federation and the cloak
Ever since the Enterprise's encounter with the bird-of-prey in 2266, the United Federation of Planets has had a strong interest in cloaking technology.
In 2268, the Federation learned that the Romulans had developed a new and improved cloaking device; one that even tracking sensors could not pick up and that posed a threat to the security of the Federation. On stardate 5027.3, Starfleet Intelligence sent the Enterprise on a covert mission across the Romulan Neutral Zone to acquire the new cloak. The mission was a success, and Starfleet was presumably able to develop a way to penetrate the new cloak. The Federation also presumably acquired a cloaking device from a captured Klingon bird-of-prey, dubbed the "HMS Bounty", in 2286. (TOS: "The Enterprise Incident"; Star Trek IV: The Voyage Home)
For reasons not entirely clear, Starfleet apparently did very little to develop cloaking technology. Then in 2311, with the signing of the Treaty of Algeron, the Federation explicitly agreed not to develop cloaking technology . The Bajoran Provisional Government also outlawed cloaking devices, stating that the possession of such a device is highly illegal. (TNG: "The Pegasus"; DS9: "Profit and Loss")
- Gene Roddenberry indicated in various interviews that "our heroes don't sneak around", indicating that the Federation made a conscious decision to not develop cloaking technology.
Following the outbreak of hostilities with the Dominion in late 2370, the Romulan Empire allowed the use of one of their own cloaks aboard the USS Defiant, although this was originally limited to usage within the Gamma Quadrant. Captain Benjamin Sisko chose to ignore this limitation on occasion. In order to ensure that the cloak was used correctly, Subcommander T'Rul was assigned to the Defiant initially, during the ship's first mission with the cloak. In exchange, Starfleet had to provide all the intelligence it received on the Dominion. (DS9: "The Search, Part I", "The Search, Part II", "Visionary")
- Ronald D. Moore commented: "We wanted to throw in a line somewhere explaining that there was a follow-on agreement with the Roms that allowed us greater freedom with the cloak, but never found a comfortable place to put it." 
After the Cardassians joined the Dominion in 2373, and flushed the Klingons and Maquis out of their territory, the Klingon High Council decided to aid the Maquis in their fight against Cardassia. They provided the Maquis with thirty class-4 cloaking devices with the Klingons' understanding that they were to be used on Maquis ships. (DS9: "Blaze of Glory")
The limited use of isolation suit technology, for anthropological purposes, may have been part of concessions made with the Romulan Star Empire during the Dominion War. However, Starfleet's use of a cloaked holoship on the Ba'ku planet was a clear violation of the Treaty of Algeron (TNG: "The Pegasus"; Star Trek: Insurrection).
- it is possible that the Isolation suits used in Insurrection were simple holographic cloaks as full invisibility screens would be unnecessary
- Speculation that the ship was provided by Section 31 would be founded, as an explanation as to how Julian Bashir was abducted for the purposes of recruitment. (DS9: "Inquisition")
Mechanics of a cloak
- Along with high power consumption, this would explain why a cloaking device and a ship's shields cannot be raised at the same time. Chang's Bird of Prey, for example, did not have its shields up while cloaked in "The Undiscovered Country".
Penetrating the cloak
Ever since the original encounter with the cloaking device, there has been an ongoing technological race between the galactic powers; as organizations like the Federation try to develop new ways of penetrating the cloak, those who use the cloak continually devise ways to render it impenetrable.The first method of penetrating a cloak was developed in 2152, when Daniels provided Enterprise with 31st century quantum beacons so they could locate a Suliban stealth-cruiser. These devices also proved effective against the Romulan cloaking devices used on mines but were ineffective against the, presumably more advanced, bird-of-prey cloaking devices. (ENT: "Shockwave", "Minefield")
The cloak used by the Romulan bird-of-prey that crossed the Neutral Zone in 2266 was less than perfect, allowing a starship to pick up a blip on its motion tracking sensors. This blip was not accurate enough for targeting of weapons and only appeared when the vessel moved, but it was enough to help the Enterprise locate the enemy vessel. By 2268, advances in Romulan technology had removed the problem, forcing Starfleet to steal a model of the new cloak. (TOS: "Balance of Terror", "The Enterprise Incident")
Versions of the cloak used by the Klingons in the late 23rd century also had their flaws. Visible energy distortions were detectable while cloaked and prior to decloaking, and in 2293, the Enterprise-A developed a method of using a torpedo to track the plasma exhaust of a cloaked ship. (Star Trek III: The Search for Spock; Star Trek VI: The Undiscovered Country)
In the 24th century, the Federation protected its borders from cloaked Romulan incursion by a gravitic sensor net. In 2368, Geordi La Forge developed a technique called the tachyon detection grid that used tachyon beams transmitted between different points to expose cloaked objects. Soon, this had been implemented on the Federation's border outposts as well as in Klingon space. (TNG: "Redemption II", "Face of the Enemy"; DS9: "Apocalypse Rising")
It is less commonly known that cloaked ships radiate a slight subspace variance at warp speeds. These variances typically vanish once the vessel drops out of warp. (DS9: "The Search, Part I") If a cloaked ship exceeds its maximum propulsion capacity, it will not be able to fully cloak, and will appear on navigational sensors as a sensor echo. (TNG: "Tin Man")
Using modified cloaks, a combined fleet of Romulan and Cardassian warships attempted to conduct a sneak attack on Founders' homeworld in 2371. They determined that as long as their fleet traveled under the speed of warp 6 their warp signatures would remain undetected, even while under cloak. This modification, however, did not prevent the fleet from being detected by the sensors aboard Deep Space 9. In this case, the cloaked fleet appeared as high concentrations of tetryon particles. (DS9: "The Die is Cast")
The Dominion, as well as the Cardassians, also possessed many methods of breaking through cloaking fields, including a long-range tachyon scanner and an antiproton beam. (DS9: "The Search, Part I", "Once More Unto the Breach") However, these methods were not always effective as Thomas Riker was able to partially counteract the antiproton beam scanning method by adjusting the cloak's resonance frequency. (DS9: "Defiant")
When Meridian appeared out of nothing in front of the eyes of the crew of the USS Defiant in 2371, it was first assumed the planet had just decloaked, until the crew realized, that Meridian was shifting from a non-corporeal dimension into the regular, corporeal universe. (DS9: "Meridian")
Improvements in cloaking technology
Firing when cloaked
Due to the enormous amount of power required to generate a cloaking field, there is by and large not enough power available to also power the weapons and shields. However, there have been several times when advances in cloaking technology have rendered this untrue.
For example, in 2293, the Klingons developed a prototype bird-of-prey capable of firing when cloaked. This ship, commanded by General Chang was used to secretly attack the IKS Kronos One in such a way that it appeared the USS Enterprise-A was responsible, implicating Enterprise commanding officer James T. Kirk in an attempted assassination of Chancellor Gorkon. Fortunately for galactic peace, the Enterprise-A was able to deduce the existence of Chang's ship and devise a way to penetrate its cloak by tracking its plasma exhaust with new sensors for analyzing gaseous anomalies. The prototype ship was destroyed by the Enterprise-A and the Excelsior over Khitomer. (Star Trek VI: The Undiscovered Country)
Another advanced cloak was encountered in 2379. The Reman warbird Scimitar employed a new type of cloaking device that allowed the ship to fire weapons and use shields while cloaked. Not only this, but the device didn't give off any tachyon emissions or residual antiprotons, making the Scimitar completely undetectable while cloaked. The effectiveness of this technology was demonstrated at the Battle of the Bassen Rift, where the Scimitar engaged the USS Enterprise-E, the IRW Valdore and another warbird, and was able to destroy one of the warbirds and cripple the other without suffering any significant damage. Only through repeated blind targeting, barrages of phasers and torpedoes, and the use of telepathic triangulation by Commander Deanna Troi was the Enterprise-E able to disable the Scimitar's cloak. (Star Trek Nemesis)
In addition, the mirror universe ISS Enterprise was able to fire weapons while cloaked using a Suliban cloaking device. However, the cloak did appear to momentarily fail a few seconds after Enterprise launched torpedoes, perhaps indicating that the Imperial ship had insufficient power for simultaneous use of weapons and cloak. (ENT: "In a Mirror, Darkly")
- In an alternate future timeline, Klingon cruisers apparently have the ability to fire while cloaked as they are able to damage the Pasteur before decloaking. (TNG: "All Good Things...")
The interphase cloak
The Treaty of Algeron did not stop Starfleet Intelligence from secretly and illegally developing the interphase cloaking device, a device which not only rendered a starship invisible, but also allowed it to pass through solid matter unimpeded. The device was lost in 2358 when the testbed vessel, the USS Pegasus was presumed destroyed by a warp core breach in a mutiny by members of the crew concerned about the legality of the test. In 2370, the Pegasus was found not to have been destroyed and the existence of the illegal device was made public to the Romulans. (TNG: "The Pegasus")
In the early 2360s, the Klingon Empire experimented with interphase cloaking technology but the research was abandoned due to several accidents.
In 2368 the Romulans also unsuccessfully experimented with phased cloaking technology, developing a prototype. The USS Enterprise-D had received a distress signal from the testbed vessel and Captain Picard ordered an away team sent over to assist. During the mission, due to a malfunction in the device, Ensign Ro Laren and Lieutenant Commander Geordi La Forge were phase-cloaked during beam-out and rematerialized on different parts of the Enterprise. After attempting to bring them back, the crew gave up, believing them dead. Eventually, Ro and La Forge succeeded in revealing what had happened to them to the crew and Lieutenant Commander Data managed to reverse the effects of the cloak, bringing them back into phase. (TNG: "The Next Phase")
Other cloaking device effects
The use of cloaking devices has occasionally led to unintended effects. The cloaking device of Aldea caused significant damage to the planet's ionosphere and ozone layer, leading to increased radiation on the surface and sterility amongst the Aldeans. (TNG: "When The Bough Breaks")
In 2371, chroniton particles generated by the Romulan cloaking device aboard the USS Defiant became lodged in the ship's ablative armor matrix. These particles interacted with the ship's transporter systems, resulting in the accidental transport of several officers backwards through time to early 21st century Earth. (DS9: "Past Tense, Part I")
Ships utilizing cloaking devices
- B'rel-class (Klingon Defense Force)
- bird-of-prey (22nd century) (Romulan Star Empire)
- bird-of-prey (23rd century) (Romulan Star Empire)
- cell ship (Suliban Cabal)
- D'deridex-class (Romulan Star Empire)
- D7 class (Romulan Star Empire)
- D12-class (Klingon Defense Force)
- K'vort-class (Klingon Defense Force)
- Negh'var-class (Klingon Defense Force)
- stealth-cruiser (Suliban Cabal)
- Valdore-type (Romulan Star Empire)
- Vor'cha-class (Klingon Defense Force)
- Defiant, USS (Federation Starfleet)
- Enterprise, ISS (Terran Empire)
- Federation holoship (Federation Starfleet)
- Klothos, IKS (Klingon Defense Force)
- Koranak (Cardassian Obsidian Order)
- Nightingale (Kraylor)
- Pegasus, USS (Federation Starfleet)
- Regent's flagship (Klingon-Cardassian Alliance)
- Scimitar (Reman)
- Aldea (Aldeans)
- Delphic Expanse spheres (Sphere Builders)
- Echo Papa 607 (Minosian)
- isolation suits (Federation Starfleet)
- self-replicating mines (Federation Starfleet)
- Hierarchy ships (Hierarchy)
- Xyrillian starship (Xyrillians)
There has been some controversy over the appearance of the cloaking device in Star Trek: Enterprise. TOS: "Balance of Terror" makes it reasonably clear that this is the first time such a device has been encountered, and early Enterprise episodes skirted very close to violating this by referring to the invisibility technology seen in ENT: "Broken Bow", "Unexpected" as stealth technology; presumably this was intended to be a primitive precursor to cloaking, utilizing some method other than the selective bending of light.
Unfortunately, the episode "Shockwave" featured several explicit references to the Suliban stealth technology as cloaking. "Minefield" further complicated the problem by showing that the Romulans had cloaking technology in the 22nd century, making some possible justifications no longer workable. Unfortunately, there is no obvious way to reconcile the discrepancies, except to force an interpretation of Spock's words ("Invisibility is theoretically possible, Captain-- selectively bending light. But the power cost is enormous. They may have solved that.") to mean that the Federation thought invisibility was impossible because all previous forms of cloaking had been penetrated. Then again, Spock's notions regarding this particular matter may not represent the totality of Starfleet's expertise, as even he admits that he is not as qualified as Mr. Scott is when it comes to engineering. (TOS: "The Devil in the Dark")
An alternative theory is that history as presented in Enterprise has been distorted by the Temporal Cold War. This could certainly explain Starfleet's early encounters with Suliban cloaking technology. As for the Romulans, it is possible that they had been given cloaks earlier than originally by agents from the future, though there is no evidence for this.
It should also be noted that the episodes of the Babel Crisis brings up the possibility that the Romulan vessel which the NX-01 encountered in "Minefield" utilized holographic technology to render itself invisible. This, however, was never directly referenced in the episodes. (ENT: "Babel One", "United", "The Aenar")
Several staff writers on Star Trek: Enterprise -- among them Producer Mike Sussman -- believed it was a mistake to give cloaking technology to 22nd Century Romulans. Season Four showrunner and Co-Executive Producer Manny Coto decided that Romulan ships would not have cloaks in any subsequent encounters.