The Breen were a reclusive, powerful, and warlike humanoid race native to the planet Breen in the Alpha Quadrant. Shrouded in mystery, the Breen were also one of the most underestimated races inhabiting that quadrant of space.
History and politics
Historically, the Klingons were among the first to discover that the Breen did not tolerate incursions into their space. During the Klingon Second Empire, Chancellor Mow'ga ordered an entire fleet of Klingon warships to invade and conquer the Breen homeworld. The fleet never returned and was never heard from again. (DS9: "'Til Death Do Us Part")
Breen privateers during the 24th century often conducted indiscriminate raids against other Alpha Quadrant species. This included the Breen attack and capture of a Cardassian Central Command vessel, the Ravinok, in 2366. The survivors of that attack were used for slave labor in Breen-operated dilithium mines on Dozaria. (DS9: "Indiscretion")
Even by the 24th century, much was still unknown about the Breen and their government, known as the Breen Confederacy. (DS9: "Strange Bedfellows", "The Dogs of War") The Federation had limited knowledge of the Breen, however, and was aware of Breen outposts located in Sector 97 in 2368. That same year, Starfleet feared the Breen might have attacked the science vessel SS Vico there. (TNG: "Hero Worship")
In 2370, the Breen participated in a palio held at Federation space station Deep Space 3, during which the Ferengi attempted to bribe a Breen pilot into deliberately losing the race. (TNG: "Interface")
In 2371, evidence seemed to briefly indicate that the Breen may have attacked another Federation space station, the Amargosa observatory, though the true aggressors were, moments later, determined to have been Romulans. (Star Trek Generations) Nevertheless, by about this point, the Breen were considered one of the most warlike species known to the Federation. (VOY: "Elogium")
In 2372, the Breen captors on Dozaria were located by Kira Nerys and Dukat. Dukat stated that he didn't approve of what the Breen were doing, by employing the survivors of the Ravinok as slaves, but did "admire their ingenuity." The Breen guards were ambushed by him and Kira, so that they could free one of the Breen's prisoners: Dukat's daughter, Tora Ziyal. (DS9: "Indiscretion") Later that year, Breen privateers raided the Bajoran colony of Free Haven; they were, however, successfully driven away by the USS Defiant. (DS9: "To the Death")
By 2373, the Cardassian Union maintained an embassy on the Breen homeworld. (DS9: "Return to Grace") The same year, Breen settlements on Portas V, near the Demilitarized Zone, dealt with the Maquis, supplying them with cold storage units. (DS9: "For the Uniform")
Whereas a Breen individual was held captive by the Dominion at Internment Camp 371 in 2373 (before killing a couple of Jem'Hadar guards there and, in retaliation, being shot to death in that year), other species like the Ferengi had developed closer ties with the Breen by the late 24th century, as both species conducted trade negotiations in 2373. (DS9: "In Purgatory's Shadow", "By Inferno's Light", "Ferengi Love Songs") Once, in 2374, Ferengi engineer Rom referred to the Breen as one of the top three toughest mercenary species that he and his brother, Quark, could find (the others being Klingons and Nausicaans). Rom suggested including the Breen in a commando team which would be sent to rescue his and Quark's mother, Ishka, who was being held prisoner by the Dominion, but Quark declined the Breen's involvement, opting for the rescue operation to be attempted by Ferengi only. Ishka's captor, Vorta clone Yelgrun, considered Breen annoying, though not as much as Ferengi. (DS9: "The Magnificent Ferengi")
It was noted by Ezri Dax in 2375, when she and Worf were captured by the Breen on the planet Goralis III, that they were a long way from Breen space. (DS9: "Penumbra") The Breen interrogated Worf and Dax, but the information which the pair consequently provided was somewhat confusing. (DS9: "'Til Death Do Us Part", "Strange Bedfellows") In Worf's opinion, the Breen were not only dangerous but also had no honor. (DS9: "'Til Death Do Us Part")
Alliance with the Dominion
- Main article: Breen-Dominion Alliance
Meanwhile, Breen representative Thot Gor had been extensively conversing over subspace with the Female Changeling, the leader of the Dominion, in discussions that she found to be particularly productive. (DS9: "Strange Bedfellows") The Breen were of no personal interest to her other than the military advantage they could provide against the Federation Alliance in the Dominion War, in which the Female Changeling was determined to lead the Dominion to victory. (DS9: "The Changing Face of Evil") In 2375, the Breen moved from a policy of low-level hostilities toward most other major powers to one of open warfare when they allied themselves with the Dominion as reinforcements. (DS9: "'Til Death Do Us Part", "Strange Bedfellows") To mark the historic moment when the two parties first met in person, the Breen presented Worf and Ezri Dax as gifts to the Dominion. (DS9: "'Til Death Do Us Part")
According to the terms of alliance, the Breen Confederacy received several planets in the Cardassian Union as compensation for joining the war. Despite being welcomed by the Female Changeling and Vorta leader Weyoun, the Breen were mistrusted by the leader of the Cardassian Union, Legate Damar, who was frustrated by the details of their treaty with the Dominion. For instance, the Breen were thereafter allowed unlimited access to the Dominion's database, whereas Damar was annoyed at them being permitted to use it at all. His frustration over how the Breen were being received led Damar to begin a Cardassian rebellion against the Dominion and enable Ezri Dax and Worf to escape back to Federation space. (DS9: "Strange Bedfellows") Even for the Cardassian rebels, the addition of the Breen made the Dominion more formidable than ever. (DS9: "The Changing Face of Evil")
Shortly after joining with the Dominion, the Breen attacked Earth, assaulting Starfleet Headquarters and destroying the nearby Golden Gate Bridge. Most of the Breen attack force was destroyed by Starfleet, though the majority of the damage on Earth had been done by that time.
The Breen were so secretive that the only thing Starfleet knew about them, up to then, was that they were a race of warriors, an aspect of the Breen which the Klingons learned in light of their attack on Earth, as not even the Klingons had ever attempted such a strike. Even Weyoun and Damar considered the Breen to be "full of surprises," albeit for different reasons. (DS9: "The Changing Face of Evil")
A short time later, the Breen assisted Dominion forces in breaking the lines at Chin'toka, launching a counteroffensive against the Federation Alliance there and breaking through in two places. To Rom's son, Starfleet Ensign Nog, the Breen now seemed unstoppable, though Worf refuted this opinion, believing that every foe was overcomeable. A major battle which followed, known as the Second Battle of Chin'toka, was won by the Dominion thanks to a unique Breen device, a kind of energy-dampening weapon which could completely drain all the main power systems of a starship. Those vessels which were destroyed by the Breen weapon during the battle included the USS Defiant. (DS9: "The Changing Face of Evil") In fact, the only ship immune to the weapon was a Klingon Bird-of-Prey, the IKS Ki'tang, which serendipitously survived thanks to an adjustment the chief engineer had made to the ship's tritium intermix in the warp core. (DS9: "When It Rains...") Under orders from the Female Changeling, the Breen allowed escape pods to carry Federation survivors to safety (believing they would sow fear in the enemy ranks after what they had witnessed), and the Breen were thanked for the decisive part they had served in the battle. (DS9: "The Changing Face of Evil") Both this defeat and the Breen's devastating assault on Earth dealt a huge blow to the Federation Alliance's morale. (DS9: "The Changing Face of Evil", "When It Rains...") The Breen were also being used to defend numerous Dominion facilities. (DS9: "When It Rains...")
Breen engineers were assigned to begin retrofitting Dominion ships with the energy-dampening weapon. The engineers were instructed to accelerate the installation program, an order that had been sent from the Female Changeling and was relayed to the engineers by Thot Pran, a Breen official. According to one report (from Kelvas V), the Breen would start installing the weapon on Jem'Hadar fighters within a week. (DS9: "Tacking Into the Wind") Although the Klingon fleet was able to make the needed adjustments against the weapon, the Federation and the Romulans were still unable to defend against the Breen. (DS9: "When It Rains...", "The Dogs of War") Hence, the Breen were prevented from acquiring final victory for the Dominion only because the Klingon vessels were present. (DS9: "When It Rains...", "Tacking Into the Wind") When the Federation finally did create a defense against the energy-dampening weapon, they forced the Dominion fleet to retreat. Thot Pran expressed an extreme distaste for surrender, and required the reassurance of the Female Changeling that, instead of surrendering, they were actually regrouping and attempting to redouble their shipbuilding and replenishment efforts. (DS9: "The Dogs of War")
With the necessary countermeasures against the Breen's energy-dampening weapon obtained, the Federation Alliance was able to go on the offensive once again, choosing to try to invade Cardassia Prime itself. In hopes of raising the Breen's will to fight, the Female Changeling offered them control of Earth and Romulus if they succeeded in winning the battle. However, she then told Weyoun this was merely a political promise, designed just as motivation. While every Breen soldier on Cardassia was participating in a concerted search for Damar, the Breen, in the interest of maintaining their alliance with the Dominion, were positioned ahead of the Jem'Hadar by the Female Changeling, under pressure from Thot Pran.
During the ensuing Battle of Cardassia, the Breen were still able to inflict major casualties on the allies, even without the use of their energy-dampening weapon. Up to a third of the allies' entire fleet was destroyed during the battle, including the Romulan flagship, even though sabotage by the Cardassian rebellion resulted in the Dominion fleet being out of contact with their headquarters until midway through the battle. The Breen continued to fight for the Dominion until the end of the war, demonstrating a willingness to die for the cause which matched that of the Jem'Hadar. However, when the Cardassian forces switched sides mid-battle, the Dominion and Breen were forced to withdraw to Cardassia Prime. The Breen leaders left Dominion Headquarters in order to fight on the front lines, and both Damar and Weyoun were shot to death shortly thereafter. At the Dominion defense perimeter in orbit of the planet, the Breen were to be targeted by the Klingons, in a three-pronged attack that saw the Romulans, Cardassians and Starfleet focus on other targets.
After Odo convinced the Female Changeling to surrender, the Breen and their allies stood down, finally bringing an end to the Dominion War. As allies of the Dominion, Breen representatives were present during the signing of the Treaty of Bajor. (DS9: "What You Leave Behind")
The Breen had no blood or other liquid circulatory system. (DS9: "In Purgatory's Shadow") The species was one of many whose thoughts and emotions were not detectable by empathic species, such as Betazoids. (TNG: "The Loss") The Breen had at least one gender: male. (DS9: "Strange Bedfellows", et al.)
The Breen constantly wore refrigeration suits. According to Worf, no outsider had ever seen what a Breen looked like under their refrigeration suits and lived. (DS9: "'Til Death Do Us Part") Even shortly before the Dominion War was concluded, their appearance remained so completely unknown to their allies that Weyoun could not avoid wondering what the Breen looked like. (DS9: "What You Leave Behind") Ezri Dax once had a dream that featured a male Breen taking off his helmet to reveal he was actually Julian Bashir, and the Female Changeling, on one occasion, commented to Weyoun that under Thot Pran's helmet was "a braver man than you." (DS9: "'Til Death Do Us Part", "What You Leave Behind")
Worf firmly concluded that the species didn't have claws, although Ezri Dax wasn't sure of this belief. She, on the other hand, speculated that the Breen might be covered in fur, due to their home planet purportedly having a remarkably cold climate. (DS9: "'Til Death Do Us Part") Dukat expected Dozaria's climate would be about fifty degrees too hot for the species, so he expressed extreme surprise at finding Breen guards on that planet. (DS9: "Indiscretion") However, while the intelligence reports available to Cardassian and Dominion leaders during the Dominion War reported their homeworld to be a frozen wasteland, Weyoun remarked (in his capacity as a diplomat) that the planet was really "quite comfortable." (DS9: "The Changing Face of Evil")
Breen appeared physically more resilient than most humanoids. When Worf struck a Breen, the Breen proceeded to incapacitate Worf with a neural truncheon, seemingly oblivious to the assault. (DS9: "'Til Death Do Us Part")
Society and culture
The Breen commonly endured pregnancy at a young age. (VOY: "Elogium") They entertained and taught their offspring with nursery rhymes. A particular example of a Breen nursery rhyme was made up of a heterophonic, five-line verse with an alternating tetrameter and pentameter structure. These were nonsensical phrases, strung together because they rhymed (at least to the Breen). Breen music which accompanied this nursery rhyme was written on a Lyxian scale. (DS9: "For the Uniform")
The Breen rank structure included the title thot, which was implied to be greater than that of a Cardassian legate. (DS9: "Strange Bedfellows") Other Breen worked as engineers, including at least four at the Kelvas facility. (DS9: "Tacking Into the Wind")
Treatment of prisoners
The Breen were known to use slave labor, and at least some of these slaves were captured in raids on other species. When the Breen were using Bajoran slaves to mine dilithium ore, there were always at least eight guards within the mines. (DS9: "Indiscretion")
The Breen were very prudent when dealing with prisoners. When capturing prisoners, the Breen typically stunned them from long range with their disruptors, rather than approaching them and risking close combat. When Worf and Ezri Dax were discovered by the Breen on Goralis while sleeping, the Breen used this technique to subdue them. The Breen also fed their prisoners a diet of algae paste. (DS9: "Penumbra", "'Til Death Do Us Part")
Aboard starships, the Breen typically sent in three guards at a time when they wished to remove a single prisoner from a cell that was being shared with other prisoners. Although Worf made various attempts at escape, they all failed, including an attempt to persuade the Breen guards that Ezri was sick, as well as an attempt to short-circuit the door of their cell. When Worf attempted to attack the Breen guards, they used a type of neural truncheon to paralyze him. Even after he fell to the ground, partially paralyzed, two Breen guards continued to use the truncheons on him to ensure he would not get back up, before two guards removed Ezri from the cell. (DS9: "'Til Death Do Us Part") In another incident, Worf simply showed signs of anger towards Weyoun and was again shocked by the neural truncheon. (DS9: "Strange Bedfellows") On the other hand, a holographic Breen, enhanced by the Hirogen and participating in Iden's Rebellion of 2377, once helped restrain B'Elanna Torres, but did so in a way that was relatively very careful, taking Torres by the arm to a position where a force field could be activated around her. (VOY: "Flesh and Blood")
To extract information from captives, the Breen used cortical implants, despite the fact that they caused extreme mental and physical side effects to the recipient, including severe pain and hallucinations. (DS9: "'Til Death Do Us Part")
See: Breen language
Science and technology
Given that most known humanoid species lived on class M planets with an average temperature far above the supposed temperature of Breen, the Breen were largely thought to have had to develop refrigeration suits in order to co-exist with others. However, if the Breen homeworld was temperate, as stated by Weyoun, then their reason for wearing environmental suits, if they were environmental suits at all, was much more mysterious. (DS9: "The Changing Face of Evil")
The Breen were known for their knowledge of sophisticated cold storage units, summed up in a statement Captain Benjamin Sisko once made: "If anyone knows how to keep things cold, it's the Breen." (DS9: "For the Uniform")
By 2373, the Breen were among multiple species known by Starfleet to use organic-based vessels. (VOY: "Scorpion") Some of their vessels were even believed to be fitted with cloaking devices. (TNG: "Hero Worship") During the mid-2370s, Breen ships utilized energy-dampening weapons. (DS9: "The Changing Face of Evil", et al.) The expediency with which the Breen could mobilize large fleets of ships surprised Damar. (DS9: "The Changing Face of Evil") The Breen's piloting skills likewise impressed Nog. (DS9: "What You Leave Behind")
The Breen were known to use type 3 disruptors, in common with the Romulans and Klingons. (Star Trek Generations) The Breen also manufactured a portable hand-held cannon, the CRM 114, which was designed to target moving objects and surface emplacements. It was among numerous weapons dealt by Ferengi arms dealer Gaila in the mid-2370s. (DS9: "Business as Usual") A holographic facsimile of a pistol was almost used by the holographic Breen in Iden's Rebellion, moments prior to that Breen being deactivated. (VOY: "Flesh and Blood")
- See: Breen space
In the Norwegian language, the term "Breen" means glacier.
Ira Steven Behr once described the Breen, considering the references to them on Star Trek: The Next Generation, as "sort of a running joke" in TNG. (Star Trek: Deep Space Nine Companion (p. 277)) Marco Palmieri similarly reasoned that contemplating all the canonical facts about Breen biology is "enough to give the impression that as each new tidbit about the Breen was being conceived, little thought was given to the composite picture being created. And it may well be that this is exactly how the Breen started." (Star Trek Magazine issue 156, p. 49)
The first evidence of the Breen was in the TNG Season 4 episode "The Loss", in which Data, during a meeting of the USS Enterprise-D's senior staff, mentions the Breen as one of "many races that are not empathically detectable." Both in that episode and the next installment to mention the Breen, TNG Season 5's "Hero Worship", the Breen are cited as a possible culprit by Data, firstly in regard to Deanna Troi losing her empathic Betazoid abilities and secondly concerning the destruction of the SS Vico. In both cases, the Breen are found not to be the cause.
Jeri Taylor proposed exploring the Breen in an ultimately abandoned season finalé that would have ended TNG Season 6.   In a memo which initially outlined the story (a document which was sent from Taylor to Michael Piller on 21 January 1992), the Breen were described as "a bellicose, brutal species who inhabit a distant but expanding empire." In the narrative, a Breen delegation was to be met and escorted to Earth by the Enterprise-D, formally establishing diplomatic relations between the Breen and the Federation. Although the Federation had long been happy that the Breen lived far from Federation space and had been expecting the species to be adversarial, the Breen had recently made the first tentative step toward establishing ties with the Federation. The unpleasantness of the species, however, became apparent to the Enterprise-D crew when the two groups met. They arranged an initial joint mission, whereby Starfleet scientists were sent on a Breen vessel to investigate a recently discovered phenomenon, but the ship suddenly vanished without explanation. In response, the Breen volatilely accused the Enterprise of being duplicitously responsible for the ship's disappearance, but the Enterprise then discovered the Breen ship, caught in an anomaly, and attempted to rescue it, the Starfleet crew jeopardizing their own vessel in the process. Jeri Taylor was of the opinion that using the story as an opportunity to introduce the Breen as a new adversary "would infuse new energy into the 7th season."  In a revised version of the plot (relayed by Taylor to Piller in a memo dated 16 February 1992), she suggested that the Breen delegation meet with the starship Indiana, under Riker's command, rather than the Enterprise, though the latter still undertook the rescue attempt (much to Riker's chagrin). When Riker opted to go to the aid of the Enterprise while it was endangered due to the anomaly, the Breen were unsettled by this decision. The volatile Breen were thereafter encountered by Captain Picard, when he came aboard the Indiana, moments before he was blown out of the ship, through a rupture in its hull, and out into space. 
Although the Breen and their involvement in the palio on DS3 are discussed as essentially smalltalk between Picard and Vice Admiral Marcus Holt in TNG Season 7 entry "Interface", the Breen's function as suspects was not finished. They weren't referenced at all in the first draft script of Star Trek Generations, despite being mentioned on-screen in the final version of that film, wherein Riker cites them as potentially responsible for the attack on the Amargosa observatory, though this is later determined not to have been the case.
When the DS9 writing staff required a villain for DS9 Season 4 episode "Indiscretion", an opportunity to physically depict the Breen presented itself. "We just struck on the idea to use the Breen," stated Robert Hewitt Wolfe. (Star Trek: Deep Space Nine Companion (p. 277))
In the script of "Indiscretion", the Breen were described as "tall, well-armed humanoids in dark, armored pressure suits which protect them from [...] harsh atmosphere." 
The hidden manner in which the Breen were portrayed – fully clothed, wearing masks and helmets – was influenced by the fact "Indiscretion" came very soon after episodes which featured large numbers of Klingons and Jem'Hadar (namely, "The Way of the Warrior" and "Hippocratic Oath" respectively). "I wasn't really in the mood to come up with a new alien race," explained Ira Steven Behr. "So I said, 'Let's not see them. Let's just put them in costume because they normally live in the cold.'" (Star Trek: Deep Space Nine Companion (p. 277)) An additional motive for Behr deciding that the Breen should appear in fully enclosed costumes was that this would cut down on make-up expenses. (Star Trek: The Official Starships Collection, issue 69, p. 17)
When the Breen helmets were being designed for "Indiscretion", it was decided that they would give viewers the suggestion of a snout, like that of an Arctic Wolf. (Star Trek: Deep Space Nine Companion (p. 277)) Terry J. Erdmann recalled about the Breen, "They were never supposed to be important at all, so when the wardrobe department first made Breen costumes, they were kind of simple; they just made a mask and put them on some guys who were going to stand in the background." (Star Trek: The Magazine Volume 1, Issue 17, p. 93)
The word "Breen" was listed in the pronunciation guide for the final draft script of DS9 Season 5 installment "Let He Who Is Without Sin...", which noted that it "rhymes with 'green'." However, the term wasn't actually used anywhere else in that teleplay. 
The Breen were not referenced in the first draft script of fifth season outing "For the Uniform", despite the species being talked about in the final version of that episode.
In the first draft script of DS9: "Sons and Daughters", the Breen were referenced, by Jake Sisko, as having fought Bel Torthap, a writer whom Jake respected. This reference to the species lasted at least two versions of the first draft script: the original (dated 1 July 1997) and a revision (dated 8 July 1997). However, the Breen weren't mentioned at all in the final draft of the script for "Sons and Daughters" (issued on 11 July 1997). 
Thus, as of the start of DS9 Season 6 (in September 1997), the show's writing staff didn't have any plans to feature the Breen in the series. However, Ronald D. Moore didn't rule out such a possibility, remarking, "We could explore them in the future." (AOL chat, 1997)
At the start of a nine-episode arc that was designed to bring an end to Star Trek: Deep Space Nine, the Breen were brought into the Dominion War by the show's writers. Thus, the species went from having been used only "a handful of times" prior to DS9 Season 7 to becoming "major players" (as worded by First Assistant Director B.C. Cameron) in the first of those installments, "Penumbra". Their introduction into the conflict was motivated by the writers feeling sure that the Breen would heighten the stakes of the war. (Star Trek: Deep Space Nine Companion (pp. 685 & 702)) Christopher L. Bennett observed that there was probably an additional reason why the Breen were introduced into the Dominion War. "I think the main reason for bringing in the Breen," he said, "was to establish that the Dominion saw Cardassia as disposable – not the partners they'd promised, but just a stepping-stone toward their conquest of the quadrant, to be tossed aside as soon as they'd outlived their usefulness (if you'll pardon the mixed metaphor). It was a motivator for Damar's – and Cardassia's – journey toward rebellion."  As the writing staff didn't have the entire war worked out when they brought the Breen into it, they were uncertain how much would ultimately be learned about the Breen in the series. (Star Trek: Deep Space Nine Companion (p. 685)) Nonetheless, the Breen ended up as the last major villains to be introduced in DS9. Also, the fact that so little had been established about them, thus far, gave Designer John Eaves a feeling of particular freeness while designing the Breen ship, for "Penumbra". (Star Trek: The Official Starships Collection, issue 69, pp. 10 & 11)
The DS9 writing staff wanted to make the Breen seem highly mysterious. "We wanted to give these guys something special," commented Ira Steven Behr. "I couldn't make them the toughest guys in the galaxy – that's the Jem'Hadar. Or the most arrogant guys – that's the Cardassians. Or the most untrustworthy guys – that's the Vorta. So we decided to make them the most mysterious guys in the galaxy, with voices that really grate on the audience." Behr has also said that the Breen's distinctive, scratchy voice, which was first spoken in "'Til Death Do Us Part", was inspired by the guitar feedback on Lou Reed's album Metal Machine Music. (Star Trek: Deep Space Nine Companion (p. 703))
When they decided to bring the Breen back, DS9's creative staff had to reuse the same costumes which had already been created for the species. "One day, the wardrobe department gets this call. 'Guess what? We're going to use the Breen,'" Terry J. Erdmann related. "Well, those masks that those poor actors are wearing have a long bill on the front, with only a tiny hole at the end for them to breathe. They snap on to the back, so they are difficult to put on and off, and the eye holes were in the wrong place, so they really couldn't see. But these almost unusable costumes were established, so they were stuck with them." (Star Trek: The Magazine Volume 1, Issue 17, p. 93) Not only did the costumes hamper breathing and seeing (the latter of which caused background extra Wade Kelly, when B.C. Cameron asked him to walk across a room, to repeatedly stumble noisily on set, during production on "Strange Bedfellows"), but wearing them also brought a host of other problems too. B.C. Cameron pointed out, "They're wearing big, clumsy boots, and their outfits are layered like an armadillo, making it very hard for them to move." Regarding the difficulties of using the Breen helmets, Steve Oster explained, "They're held together with magnets and they fall off any time someone bumps them. And before we redesigned them, the switches for the little blinky lights were on the inside of the helmet." This meant the helmets had to be removed from the actors every time the lights needed to be turned on. "And for some reason we never did figure out," continued Oster, "the nine-volt batteries only lasted minutes before they burned out." Thot Gor actor Todd Slayton considered "the biggest problem with the Breen mask" was the tiny hole for breathing, which he estimated was about eight inches away from the wearer's nose. He also noted that the viewing lens in the mask often steamed up quite quickly. (Star Trek: Deep Space Nine Companion (p. 702))
During the course of writing the Dominion War, the DS9 writers were very intent on establishing the Breen as extremely formidable, tough aliens. This inspired the introduction of the Breen energy-dampening weapon and the destruction of the USS Defiant, in "The Changing Face of Evil". (Star Trek: Deep Space Nine Companion (p. 693))
The final Breen to be depicted (both in the show's chronology and production-wise) was the holographic individual in VOY: "Flesh and Blood". The final draft script of "Flesh and Blood, Part I" called for multiple holographic Breen to appear in the episode, although only one is present in the final version.
Uncertainties and trivia
Ultimately, it was never made clear how the Breen actually looked, inside their costumes. Whereas First Assistant Director Lou Race "always" suspected the Breen looked similar to Donald Duck (owing to the beak-like part of their helmets), Make-Up Supervisor Michael Westmore admitted that he wasn't really sure how they looked. However, he also reflected, "I actually had an unused head in the lab that would have been great. I thought we could put a little bit of a snout onto it, so we could justify the snout on the helmet. But we never had the opportunity to show one of them." (Star Trek: Deep Space Nine Companion (pp. 701-702))
It was also left unclear whether anyone had ever seen a Breen and survived the experience. In "'Til Death Do Us Part", Worf states, "No one has ever seen [a Breen] and lived to speak of it." However, "Indiscretion" implies that Kira and Dukat must have gotten a look at a pair of Breen whose uniforms they stole. Compounding the continuity problem, Kira is implied as having dispatched another Breen, again for his uniform, in DS9 series finalé "What You Leave Behind". In hindsight, Ronald D. Moore regarded this continuity glitch as one of numerous frustrating details that, due to their sheer quantity, the creative team hadn't managed to keep track of. On the other hand, Ira Steven Behr proposed an in-universe explanation, saying, "There's nothing in those helmets. I don't think there's a guy in there, which is something we never got around to saying." After a pause, he added, "Or maybe there's a little slug, some tiny little creature in there. I never wanted them to be humanoid in any way." (Star Trek: Deep Space Nine Companion (p. 702)) Despite this, not only did the aforementioned script of "Indiscretion" regard the Breen as "humanoid" but so do the reference works Star Trek Encyclopedia (3rd ed., p. 54) and Star Trek: Star Charts (p. 35) as well as the StarTrek.com database entry about the species. 
The writers of Star Trek: The Official Starships Collection (issue 69, pp. 16 & 17) approved of the Breen. They regarded them as "vitally important to the Dominion War" and further remarked, "Despite the hazards of the costume, the Breen proved a tough and formidable adversary that ratcheted up the threat to the Federation in the final months of the Dominion War. Without them, the final story arc would not have been so tense and compelling."
Marco Palmieri wrote, "You have to hand it to the Breen: For a civilization that started out as a sometimes-mentioned but never seen running gag on The Next Generation, they eventually came incredibly close to handing the Founders a decisive victory in the Dominion War. Along the way, the Breen not only destroyed the Starship Defiant, they succeeded where no other enemy empire had before: attacking Starfleet Headquarters on Earth. Even the Klingons had reason to fear them [...] [considering] an Imperial fleet sent to conquer the Breen homeworld was never heard from again. That's not a bad résumé as Star Trek villains go." (Star Trek Magazine issue 156, p. 48)
Glenn Hetrick of Star Trek: Discovery has expressed an interest in the Breen appearing in that series without their masks. Hetrick commented, "No one in the universe, as per Worf, has ever seen what they look like under the refrigeration suits. So, that is definitely – as many times as I can put it forth – I want to get one of those refrigeration suits off and see what the Breen look like." 
In the novel Zero Sum Game, the Breen are revealed to be comprised of several different species, with "Breen" as the name of a society, rather than a single species. The primary reason for this is to judge individuals on their skills and abilities rather than their physical appearance. In reality, however, this approach was taken by Zero Sum Game author David Mack because he believed that trying to reconcile all the canonical "facts" about the Breen's physiology would otherwise be impossible, as he thought there were too many discrepancies. (Star Trek Magazine issue 156, p. 50)
Among the species in the Breen Confederacy mentioned in Zero Sum Game, only four were mentioned by name and described in some detail, and an additional fifth species was described in the novel Plagues of Night. Two more species were named in the novels Disavowed and The Hall of Heroes. They include:
- The Silwaan (β), a humanoid species and a founding species of the Confederacy, who possessed (in the case of the character Chot Nar) bronze-tinted skin, white hair, and jade-colored eyes.
- The Fenrisal (β), a furred, lupine humanoid species whose snouted facial structure served as the inspiration for the archetypical Breen helmet design.
- The Paclu (β), a large, powerful humanoid species who often served in the Confederacy military thanks to their strength. They are noted to possess four-lobed brains that make their minds difficult to read, as well as an unspecified number of hearts.
- The Amoniri (β), a humanoid species whose bodies possessed no blood and evaporated when exposed to normal M-class atmospheres, requiring them to wear actual refrigeration suits to function outside of their regular environment. They also served commonly in the Confederacy military alongside the Paclu.
- The Vironat (β), a humanoid species with cleft arms and legs with highly accurate sensory organs located along their lengths that granted them exceptional tactile abilities, making them highly effective engineers. The enhanced sensory abilities of the species also rendered them prone to motion sickness.
- The Silgov (β), a humanoid species that can easily mingle among the peoples of the Federation. Author David Mack has said that this is actually a mistake, which is meant to refer to the Silwaan. 
- The Kalystarians (β), a humanoid species with no nose and a pale, scaly complexion.
Decipher's Star Trek Roleplaying Game supplement Aliens explained the discrepancy over whether anyone had seen a Breen and lived by establishing that the Breen disintegrate upon exposure to atmosphere.
The video game Star Trek: Conquest features Breen as one of the people groups which players can assume the role of. Of the three types of commanders available – attack, defense, and movement – the Breen only have defense (2) and movement (1) available.
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