The Breen's true appearance remained unknown to viewers until the 2024 fifth season of Star Trek: Discovery episode "Mirrors" while flashing back to reveal the history of the character L’ak.

According to DS9 writer/producer Robert Hewitt Wolfe, after having previously been treated as red herrings since their first mention on Next Generation, and as a running joke by the production staff because "they were these people who were out there who were dangerous but were never really responsible for any of the trouble going on", they would finally be used as the villains in an episode.

Their true appearance would be concealed beneath masks, according to writer/producer Ira Steven Behr, because "I wasn't really in the mood to come up with a new alien race.

[citation needed] In the Star Trek: Lower Decks season 3 episode "Trusted Sources," the USS Cerritos encountered Breen occupying the planet Brekka while on a second contact mission.

In a possible future witnessed by Rayner and Michael Burnham, the Breen were sold the technology of the Progenitors by L'ak and Moll and they used it to destroy the Federation.

L'ak, who was considered an outcast despite being the nephew of the Ruhn Primarch of the Sixth Flight, had abandoned his people after falling in love with Moll.

L'ak, as the last direct descendant of the Breen Emperor, is a crucial political pawn as whoever controls him hold legitimacy in the war for the throne.

Moll kills Ruhn and takes control of his forces with the intention to resurrect L'ak and place him on the throne of the Breen Imperium.

Finding a structure containing a portal to the technology close to binary prime black holes, the dreadnaught seizes it, forcing a Discovery away team to sneak aboard the ship in replicated Breen refrigeration suits and steal it.

Unable to destroy the dreadnaught, Rayner instead uses Discovery's spore drive to send it and a scout ship sent by Tahal to the galactic barrier.

[citation needed] In Zero Sum Game, the first novel in the 2010–2012 series Star Trek: Typhon Pact, writer David Alan Mack establishes that the Breen Confederacy is made up of many separate species, all of which wear suits to disguise their respective appearances and present as a unified Breen identity.

[citation needed] In Star Trek Generations Riker mentions the Breen as one of only three species with access to Type-III hand-held disruptors.

[citation needed] In an alternate timeline witnessed in the "Face the Strange" episode of Star Trek: Discovery's fifth season, the Breen were sold the technology of the Progenitors, a race that had seeded the galaxy with humanoid life, and they used it to wipe out the Federation.

[citation needed] In Star Trek: Discovery's fifth season, the Breen have massive dreadnaught-class ships with shields so powerful that they can block the Federation's scanners.

[citation needed] A Breen participated in the palio on Deep Space 3, according to "Interface", a 1993 seventh season Next Generation episode.

[5] Breen privateers attacked Free Haven, a Bajoran colony, in "To the Death", a 1996 fourth season Deep Space Nine episode.

[citation needed] A Breen appeared on Star Trek: Voyager as a hologram enhanced by the Hirogen, in the seventh-season episode "Flesh and Blood".

[citation needed] The Breen function as a common recurring enemy for the Federation and the Klingon Empire in Star Trek: Online.

[citation needed] A Breen is one of the main characters in Star Trek: Renegades, a fan film posted to YouTube.