When the second Flash, Barry Allen, decided to retire, he journeyed to the 30th century with his wife, Iris. There, they had two children, Don and Dawn Allen, who inherited Barry's super speed and connection to the Speed Force. When the Flash was called back to the past to fight in the Crisis on Infinite Earths, in which he died saving the universe, Iris raised their kids alone, and they grew up to be the heroic Tornado Twins. Don Allen grew up and married Meloni Thawne, daughter of the president of Earth, but also a descendant of Flash villains Cobalt Blue and Professor Zoom. Their son Bartholomew, or Bart for short, was born with a strong connection to the legacy of all speedsters, both good and bad.
Bart exhibited speed powers from birth, and not only did he have a super high metabolism, but he aged at hyperspeed as well. Although he was chronologically only a toddler, physically, he looked like a pre-teen. When his father Don was killed by the alien race known as the Dominators, they kidnapped Bart, with the intention of studying his super speed powers. He was then raised in a virtual reality environment, inside of which his mental age matched his physical one. With his super speed out of control, and threatening to kill him of old age by the time he was a few years old, his grandmother Iris West took stole him away, and brought him back to the present. It was her hope that her nephew, the third Flash Wally West, could teach Bart how to control his speed, and therefore save his life.
Wally was able to help him control his speed and slow down his aging, and now at the age of approximately a young teenager, Bart took on the name Impulse, and became the partner of the Flash, much as Wally had been Barry's sidekick. During this time, he also briefly joined the Titans, Wally's former team. Wally suggested that Bart needed to learn to slow down, so he suggested that Bart move to Manchester, Alabama, hoping the slow pace of life there would make him a little less hyper. While in Alabama, he was tutored by the Golden Age speedster Max Mercury, who became like a surrogate father to him. Max helped train Bart in controlling his powers, and when he vanished into the Speed Force, Bart was taken in by Jay Garrick, the original Flash, cementing his ties to several generations of speedsters.
Bart made friends for life when he met Superboy, the Tim Drake Robin and Wonder Girl Cassie Sandsmark, and together they became the teen team called Young Justice. The team had many adventures, and essentially grew up together. Eventually Young Justice disbanded, but it wasn't long before they reformed as the Teen Titans. A cocky Impulse then had his kneecap blown out by the mercenary Deathstroke, but later he replaced it with an artificial one. During his recovery, Bart read every single book in the San Francisco Public Library at super speed, absorbing all the knowledge within. Seeing this as a rite of passage, he adopted the costume and name of Kid Flash, as an homage to his mentor Wally West.
During the cosmic event known as the Infinite Crisis, the villainous Superboy-Prime attacked Bart's friend, the Conner Kent Superboy, during a brutal rampage. Bart joined with Flashes Wally West and Jay Garrick to put an end to the threat of Superboy-Prime, by running him at full throttle straight into the Speed Force. Jay found he simply could not run fast enough to enter the Speed Force, and Wally became energy and disappeared. Now alone with Superboy-Prime, Bart was assisted by his grandfather Barry Allen, along with other dead or missing speedsters like Johnny Quick and Max Mercury, who appeared and took Superboy-Prime into the Speed Force, along with Bart. Jay then believed the Speed Force was gone. But Superboy-Prime managed to escape the Speed Force, and he was pursued by Bart, who had aged four years while inside it, and was now the age equivalent of a man in his early twenties.
Attempting a normal life without super heroics, Bart got a job with Keystone Motors, but was still connected to the Speed Force, which was actually not destroyed. However, he soon discovered that his Speed Force connection was killing him, similarly to how it had been when he was a child. Scientists at S.T.A.R. Labs then modified one of his grandfather Barry Allen's old uniforms in an effort to help him control his Speed Force connection, and Bart now became the fourth Flash. Following even further in his grandfather's footsteps, he moves to California and began working in a police forensics lab.
Bart's career as the Flash was tragically short-lived however. Bart's evil clone counterpart Inertia worked together with the Flash's Rogues, who were duped into helping Inertia build a machine that would transfer the Speed Force from Bart into himself. The machine removed his powers, but in doing so, it threatened to overload into a massive explosion that would kill millions. Bart then sacrificed himself to stop the machine, and the Rogues had killed their first Flash.
Bart's death coincided with Wally West's return from the Speed Force, and it is theorized that the moment Wally returned was the same moment that Bart lost his powers, and Wally absorbed the newly released Speed Force. The Rogues then killed Inertia for being tricked into helping them kill a Flash. But Bart does not stay dead for long, as the in the 31st Century, the Legion of Super-Heroes' Brainiac 5 uses special living lightning rods to draw out the essence of Bart from the Speed Force within Wally, and when Bart is released, he is a teenager again, and once again in his Kid Flash persona. He returns to the past where he resumes his heroic career and rejoins the Teen Titans.
In the post-FLASHPOINT universe, a different version of Bart Allen appears, and is a member of the Teen Titans. But it is soon revealed that this "Bart" isn't really Bart at all. His real name turns out to be Bar Torr, a speedster sent to the 21st century as part of a witness protection program, where he is to stay until he is to return to the future to stand trial for several crimes, including killing members of the oppressive regime which murdered his parents.
Bar Torr is given the false name of Bart Allen, along with false memories, when he is sent to the past. Although he returns to his future timeline, he eventually comes back to the 21st Century to rejoin his friends in the Titans. After the events of DC UNIVERSE REBIRTH #1, Bar Torr's status in the DC universe is unknown. But after the events of the so-called Flash War between Wally West and Barry Allen, the true Bart Allen is released from the Speed Force, back in his Impulse costume once more.