Star Trek's Gamma Quadrant, first introduced in Deep Space Nine (1993–1999), presents the crew of the titular outpost with a whole new place to boldly go. In the show's pilot, Starfleet's Commander Benjamin Sisko (Avery Brooks) discovers a rare stable wormhole linking the friendly world of Bajor to the galaxy's distant Gamma Quadrant.
As a result of his discovery, Sisko becomes a pivotal figure in Bajoran mythology (Emissary to the Prophets) and sets in motion a chain of events that will threaten the Federation's existence. However, as well as danger, the Gamma Quadrant offers many chances for exploration, commerce, and diplomacy with the numerous species who call it home.
10 Wadi
Although the Wadi's debut episode is hardly a favorite of many fans, these tattooed aliens are, notably, among the first Gamma Quadrant residents to visit Deep Space Nine. Their single-minded interest in gambling results in the Wadi trapping Sisko inside their immersive board game, Chula. Despite their less-than-classic origins, the Wadi and their technology are an interesting precursor to VR gaming today.
The Wadi would later reappear in Star Trek: Lower Decks, apparently not having altered their ways, as Ensign Boimler (Jack Quaid) berates them for always "trapping people inside of games". Some things never change.
9 Yrythny
Following DS9's finale, the series spawned a range of sequel novels that would grow, over the next two decades, to encompass the entire franchise. The Mission Gamma arc concerns the Federation's renewed exploration of the Gamma Quadrant, as the crew of the Starship Defiant encounter the Yrythny, aquatic aliens divided into two conflicting castes.
Federation intervention helps the Yrythny to reform their divided society and to counter aggressive neighbors. In return, the Yrythny's unique genetic makeup proves helpful in improving the waning fertility of the Alpha Quadrant's Andorians.
8 Saltah'na
The Saltah'na are one of the many Gamma Quadrant races who don't appear in the flesh in DS9. Destroyed in a conflict long ago, they remain preserved in a telepathic matrix. When the matrix arrives at DS9, the crew find themselves taking on the personalities of the dead race and re-enacting a centuries-old power struggle.
Despite lacking corporeal forms, the Saltah'na are able to exploit the strained relationship between Sisko and his Bajoran liaison Kira (Nana Visitor). It is only the intervention of Odo (René Auberjonois), the station's shapeshifting constable, that prevents the Saltah'na from repeating their own annihilation.
7 Argrathi
It's a running joke among DS9 fans that each season features an episode where lovable everyman Miles O'Brien (Colm Meaney) undergoes a traumatic experience. In "Hard Time", it is the Argrathi who are responsible. Instead of a conventional prison system, the Argrathi rely on technology that implants false memories of incarceration. O'Brien receives 20 years of mental imprisonment after being suspected of espionage.
Despite their brutal justice system, the Argrathi are not straightforward villains. Once they realize O'Brien's innocence, he is freed. However, the memories of O'Brien's incarceration continue to haunt him.
6 Yaderans
DS9 creates anticipation for the mysterious Dominion by showing their impact on other Gamma Quadrant civilizations. One such race are the Yaderans, human-like aliens who were forced to abandon their homeworld after its annexation by the Dominion.
After encountering the species, Odo and science officer Jadzia Dax (Terry Farrell) learn that most of the Yaderans are in fact holograms, produced by the planet's single occupant to re-create his lost world. Although the holograms are vanishing due to technical problems, Dax is able to repair the projector, preserving the simulacrum.
5 Paradans
One of many Gamma Quadrant species responsible for giving O'Brien a bad day, the Paradans are able to create replicants so convincing that even the replicants themselves are unaware of their true nature. In "Whispers", the Paradan government replicates O'Brien and programs him to assassinate a key figure at a peace conference.
Although the attempt is foiled, the replicant O'Brien is killed. Later, Lower Decks would show a statue commemorating O'Brien's role as "perhaps the most important person in Starfleet history". It's safe to say that he earned it.
4 Hur'q
Little is revealed about the Hur'q in DS9, except that they invaded the Klingon homeworld 1000 years before, stealing the legendary Sword of Kahless. Even the actual name of the species remains shrouded in mystery, with "Hur'q" simply being a Klingon word used to refer to outsiders.
The Hur'q are expanded upon in Star Trek: Online, in which the species reawakens from its centuries-long hibernation. Despite their lack of screen time, the Hur'q were powerful enough to take on the Klingon Empire and win, making them a race deserving of more on-screen exploration, perhaps in the rumored Star Trek: Legacy series.
3 Jem'Hadar
Genetically engineered to serve the Dominion's Founders, most Jem'Hadar are loyal to the point of self-destruction, as evidenced when a Jem'Hadar ship rams the Galaxy-class USS Odyssey. The destruction of a ship identical to TNG's Enterprise-D (the same hollow model was used for filming) establishes the Jem'Hadar as a deadly threat to the status quo.
Later stories bring nuance to the species, with Doctor Bashir (Alexander Siddig) attempting to cure the alien warriors of their dependence on the Founder's drugs, while those naturally immune to the addiction would attempt to overthrow the Dominion.
2 Vorta
The bureaucratic Vorta are best personified by Weyoun (Jeffrey Combs), the slimy yet charming Dominion diplomat. Star Trek's writers were so impressed by Combs' portrayal that they developed the Vorta as a race of clones in order to bring the actor back after Weyoun's initial demise.
Second only in importance to the Founders, who the Vorta view as gods, the Vorta maintain the Dominion's status as a Gamma Quadrant superpower. They also number a celebrity cameo among their ranks: after being injured in a mosh-pit, Iggy Pop took time away from his rockstar lifestyle to play the Vorta Yelgrund.
1 Changelings
The Changelings, also known as the Founders, are the Dominion's ruling intelligence. Their shape-shifting nature makes them terrifying adversaries: just four Changelings are able to plunge Earth into chaos in DS9's fourth season. The Changeling's sense of superiority over "solids" and their ability to play the Alpha Quadrant's powers off against one another makes them one of the Federation's greatest foes—even before they commence their full-scale invasion.
The evolved Changelings that appear in Star Trek: Picard are the result of meddling by Section 31, Starfleet's not-so-secret society, and update the classic Gamma Quadrant race for a new era of Star Trek. The Changelings remain, ironically, one of Star Trek's most solid concepts.