The Flash is an American superhero television series developed by Greg Berlanti, Andrew Kreisberg, and Geoff Johns, airing on The CW. It is based on the Barry Allen incarnation of DC Comics character the Flash, a costumed superhero crime-fighter with the power to move at superhuman speeds. It is a spin-off of Arrow, existing in the same fictional universe known as the Arrowverse. The series premiered in the United States on The CW on October 7, 2014, and ran for nine seasons until May 24, 2023. The series follows Barry Allen, portrayed by Grant Gustin, a crime scene investigator who gains super-human speed, which he uses to fight criminals, along with others who have also gained superhuman abilities.

Initially envisioned as a backdoor pilot, the positive reception Gustin received during two appearances as Barry on Arrow led to executives choosing to develop a full pilot to make use of a larger budget and help flesh out Barry's world in more detail. The series is primarily filmed in Vancouver, British Columbia, Canada.

The Flash's premiere on October 7, 2014 became the second-most watched pilot in the history of The CW, after The Vampire Diaries in 2009. It has been well received by critics and audiences, and won the People's Choice Award for "Favorite New TV Drama" in 2014. The series, together with Arrow, has spun characters out to their own show, Legends of Tomorrow, which premiered on January 21, 2016.

The first season follows Barry Allen, a crime-scene investigator who gains superhuman speed from the explosion of the S.T.A.R. Labs' particle accelerator and becomes the costumed superhero the Flash. He uses his new powers to fight criminals along with other metahumans who have also gained superhuman abilities in Central City. Barry eventually discovers that his mentor Harrison Wells is actually Eobard Thawne–the Reverse-Flash–his archenemy from the future who traveled back in time to murder his mother, Nora Allen, when he was a child. Thawne is ultimately erased from existence when his ancestor Eddie sacrifices himself, causing a singularity to form in the process.

In the second season, the singularity brings the speedster Zoom from a parallel universe of Earth-2, who seeks to eliminate all other speedsters in the multiverse where Barry meets his father Henry Allen's Earth-3 counterpart Jay Garrick. After Zoom kills Barry's father, Barry defeats Zoom and travels back in time to save his mother's life.

In the third season, Barry creates the alternate timeline "Flashpoint" following his decision to change his past. He is able to partially restore the timeline, but causes the emergence of Savitar, a god-like speedster with a grudge against Barry. When Barry accidentally travels to the future and sees Iris West killed by Savitar, he becomes desperate to change the future to prevent that from happening. After saving Iris and defeating Savitar, Barry takes his place in the Speed Force in order to repent for his creation of Flashpoint.

The fourth season sees the team successfully bring Barry back from the Speed Force, but in the process release dark matter that turns a dozen people on a city bus into metahumans, masterminded by Clifford DeVoe, an adversary with the fastest mind alive. After the defeat of DeVoe, the team is approached by Barry and Iris' daughter from the future Nora West-Allen.

During the fifth season, the team discover that Nora's presence has altered the timeline and unleashed Cicada, a serial killer bent on killing metahumans, as well as the aftermath of thwarting Thinker's plot causing ordinary items to be imbued with dark matter that turns them into Meta-Tech. They also eventually learn of her allegiance with an imprisoned Eobard, who orchestrated Nora's arrival and previously trained her when it came to her fight with Godspeed. Barry and Nora succeed in subduing an escaped Eobard, but are forced to let him go and Nora is erased from the timeline.