“To seek out new life and new civilizations.” The phrase is part of the ethos of all that is Star Trek. Of course, right before it in the opening narration there's the even more famous use of three oft-repeated words: “strange new worlds.”
Gene Roddenberry and the original producers of Star Trek were far too polite to cast aspersions on any of our cousins from beyond the farthest star – but what they really were implying was this: weird aliens.

For years we never even saw a Breen. (Hey, we're still waiting to see a Tzenkethi!) Yet the name was dropped here and there as far back as the fourth season of TNG. All we really learned was that they were unaligned miscreants you didn't want to get involved with. (Even the Klingons knew well enough to leave them alone, so that ought to tell you something.) When they eventually got their day as on-air aliens during DS9's fourth season, producer Ira Steven Behr let exhaustion be the mother of brilliance.
“I wasn't really in the mood to come up with another alien race,” he's quoted as saying, so it was decided to merely stick a helmet on them instead of designing a new creature design. And this made them AWESOME.

Even though nearly every other alien speaks English on Star Trek (of course, we the audience are fitted with a Universal Translator, let's not forget) the Breen speak in dissonant electronic warbles. Behr has stated that the Breen's speaking patterns are based off of the chaotic sounds from Lou Reed's notorious, contractual obligation avant-noise album “Metal Machine Music.”
Only hints of Breen culture are known in canon. The suits supposedly keep them at extreme cold temperatures. They have no blood or other circulatory fluids. Much like Ferengi, empaths are unable to read their feelings. They are ruthless, to the point that the Romulans have a saying that goes “never turn your back on a Breen.”
The usually isolationist Breen joined the Dominion after the morphogenic virus had been introduced to the Founders' Great Link. With a need to end the Dominion War quickly, the Founders promised the Breen several Cardassian and Federation planets at the conclusion of the war, though an aside from the Female Changeling to Weyoun implied that they may have been planning to go back on their deal.
The Breen's advanced weapons did major damage to Earth, heaping considerable damage upon San Francisco. They also fought alongside the Dominion during the Second Battle of Chin'toka, and their advanced weapons, namely the Energy Dissipator, leveled Federation and Romulan ships. (Luckily, many Klingon vessels were able to survive.)

There's a theory that the Breen's alliance with the Dominion is actually more Founder subterfuge. In “In Purgatory's Shadow,” when Garak and Worf come to Internment Camp 371, there is a rather tame Breen there alongside Martok and Dr. Bashir. One could interpret this to mean that this was a high-ranking Breen who, like the others, had a Changeling in its place, bending fate to the Dominions' whim.
And that's what's so great about the Breen is that we'll never really know for sure.
For those that go deep, David Mack's book from 2010, Zero Sum Game, is a big, fat wet kiss for Breen fans. In it he digs into (non-canon, of course) Breen culture, explaining that the Breen are actually made up of multiple species hailing from the same planet who appear the same to us because of their refrigeration armor.
To me, the complete distance with which the Breen is kept from the audience is something of a breath of fresh air, in a way. In Star Trek, no one is all evil. Even the Jem'Hadar, who are BRED to be killing machines, are shown in a light that captures our sympathy. Their dependence on Ketracel-white, it is implied, is all that's keeping them mindless instruments of violence.

The Breen are seen fighting alongside the Dominion right until the Cardassians break ranks and the Founders have their change of heart. The Vorta and Jem'Hadar, naturally, just follow suit as the Founders sign the peace treaty with the Federation. But the Breen? Well... we never really see what becomes of them. They're still out there in their refrigeration suits and badass helmets, one of the most wonderfully weird aliens of the galaxy. Who knows when they plan to pay a visit to us next?
Do you have any top Breen memories? Let us know what you think about this curious species in the comments below.
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Jordan Hoffman is a writer, critic and lapsed filmmaker living in New York City. His work can also be seen on Film.com, ScreenCrush and Badass Digest. On his BLOG, Jordan has reviewed all 727 Trek episodes and films, most of the comics and some of the novels.