(covers information from several alternate timelines)
The Beta Quadrant was the common designation during the 24th century for one-quarter of the Milky Way Galaxy when viewed from a point of view perpendicular to the plane of the galaxy. It extended ninety degrees spinward of an imaginary line drawn from the center of the galaxy, the galactic core, passing at least near the Sol system. (VOY: "Pathfinder", "Year of Hell")
Major portions of the Klingon Empire and the Romulan Star Empire were located in the Beta Quadrant. The Federation also claimed territory in the quadrant in the 23rd and 24th centuries. (Star Trek: Insurrection production artwork)
In 2293, the starship USS Excelsior, commanded by Captain Hikaru Sulu, completed a three-year exploratory voyage in the Beta Quadrant which included cataloging gaseous anomalies. (Star Trek VI: The Undiscovered Country)
Seventy years later, Captain Lisa Cusak and the crew of the USS Olympia began a seven-year exploration voyage in the Beta Quadrant. Following this mission, the Olympia crashed on a planet in the Rutharian sector en route back to Federation space. While escorting a convoy to the Vegan system in 2374, the crew of the USS Defiant made contact with Cusak in 2371 via a temporal anomaly. (DS9: "The Sound of Her Voice")
In an alternate timeline, the USS Voyager used a quantum slipstream drive to cross the Delta and Beta Quadrants, before crashing on an Class L planet in the Takara sector, a few parsecs short of reaching the Alpha Quadrant. Also, Starfleet Intelligence salvaged a Borg temporal transmitter from the debris of a Borg cube in the Beta Quadrant. (VOY: "Timeless")
A Borg ship, commandeered by liberated drones and Axum, was located in a remote sector of the Beta Quadrant. Axum described this location as being on the border of fluidic space. (VOY: "Unimatrix Zero, Part II")
When members of the Hierarchy captured Captain Janeway and forced The Doctor to obtain Voyager's warp core for them, he impersonated Janeway and told Commander Chakotay a cover story involving a fictitious R'Kaal Imperium which supposedly controlled "thousands of parsecs from here to the edge of the Beta Quadrant." (VOY: "Renaissance Man")
- Very little is known about the Beta Quadrant, canon or otherwise. Graphics from VOY: "Pathfinder", "Year of Hell", and "Endgame" support the location of the Beta Quadrant relative to the other quadrants. That the Sol system lies at the border between the Alpha and Beta Quadrants is stated explicitly in the Star Trek Encyclopedia, and is supported in part by a graphic from "Year of Hell, Part I".
- Although the Star Trek Encyclopedia states that Federation, Klingon, and Romulan space are all situated across both the Alpha and Beta Quadrants, all three powers and governments in their vicinities are exclusively referred to as being in the Alpha Quadrant on Star Trek: Deep Space Nine and Star Trek: Voyager. For example, the Dominion War is characterized as a conflict between powers of the Alpha and Gamma Quadrants. Similarly, Voyager's destination is the Alpha Quadrant, not whatever territory the Federation or its allies may claim in the Beta Quadrant. Throughout both series, virtually every race that is not explicitly located in the Gamma or Delta Quadrants is described as indigenous to the Alpha Quadrant.
- A possible rationalization for this discrepancy is that characters use the term "Alpha Quadrant" as shorthand for the group of governments that collectively reside at least partly in the Alpha Quadrant. (This would be similar to how Russia and Turkey may be described as European nations.) However, the extent to which these governments' territories extend beyond the Alpha Quadrant (if at all) has never been established in canon.
- The first serious references to the Beta Quadrant were towards the end of Star Trek: Voyager, more an indication that the crew's journey was relatively close to an end than anything else. Nevertheless, fans have claimed that, due to the distance traversed by the USS Voyager, that series should have seen the characters reach the Beta Quadrant before the end of the seven year run.
- The Fen Domar, with whom Voyager was engaged in battle in an alternate timeline, may be a Beta Quadrant civilization. This is based on the ship's close proximity to the quadrant's border in "Endgame" and Admiral Kathryn Janeway's comments that the ship would have encountered them just a few years after the events of that episode.
- According to scenic artist Geoffrey Mandel's book, Star Trek: Star Charts, the Beta Quadrant was also home to the Vulcans, Andorians, Orions, Bolians, Son'a, Risians, Organians, Coridan, Axanar, Nausicaans, Suliban, Gorn and a litany of Star Trek's more recognizable alien species. In fact, the book suggests that most of the early voyages of the Enterprise NX-01 took place in the Beta Quadrant with only a few excursions into the Alpha Quadrant. This is, however, not necessarily accurate, as while the book was created to strictly adhere to canon, episodes aired after its publication sometimes contradict its content.