- Regardless of the cost, I don't fail you... And no matter how difficult the target is to hit, I don't miss.
- -- Clint Barton
A former Olympic archer, Clint Barton was freed from death row and became a U.S. intelligence agent over a decade ago. Active around the world, he was present for the liberations of Kosovo and Afghanistan, as well as the fall of the Berlin Wall. He became a close friend and frequent colleague of fellow agent Nick Fury, and the two operatives were key founders of the modern intelligence agency S.H.I.E.L.D.. Both rose through the ranks until Fury became the agency's director and Barton (code-named Hawkeye) became an elite agent of S.H.I.E.L.D.'s black ops division, often partnered with ex-KGB espionage legend Natasha Romanov (code-named Black Widow). Despite his dangerous, globe-spanning profession, Barton managed to build a more stable family life than most of his peers, residing for years with his girlfriend Laura (who recently became his wife), with whom he had three children: Callum, Lewis and baby Nicole, named after her godfather, Nick Fury. Hawkeye habitually phoned his family before every mission to say good-bye, just in case he never returned; but he always returned, even after a mission which pitted him, the Black Widow and their support team against two office buildings full of extraterrestrial Chitaurisleeper agents. Most of their team died, but Hawkeye, the Widow and a handful of others survived, wiping out the Chitauri sleepers in the process.
Shortly thereafter, Hawkeye and the Widow were promoted to membership in S.H.I.E.L.D.'s new celebrity superhero team, the Ultimates, complete with newly falsified backgrounds to make them more acceptable public figures. When the Ultimates thwarted a Chitauri plot to destroy Earth, thanks in part to the unwitting assistance from the monstrous Hulk, it was a wounded Hawkeye who managed to bring down the rampaging Hulk once the aliens were defeated. Hawkeye participated in many more Ultimates' operations before taking part in the team's most controversial mission to date, teaming with the European Super-Soldier Initiative to strip a "rogue" state of its nuclear weapons capability. In response, a traitor within the Ultimates led a black ops team to Hawkeye's home, slaughtering the entire Barton family. Hawkeye himself was taken alive and chemically interrogated, yielding secrets which enabled a foreign super-army to overwhelm the Ultimates. Hawkeye soon escaped from the prison using his fingernails as a weapon. When the traitor within the Ultimates was revealed to be the Black Widow, Hawkeye killed her in revenge for the death of his family.
Hawkeye became reckless and possibly suicidal. When the Ultimates (newly relocated to Tony Stark's mansion) were attacked by someone seeming to be Venom, Hawkeye attacked without waiting for his teammates, and seemed to welcome being nearly killed. He also became more violent, expressing glee as he killed various dupes of Madrox when the Brotherhood of Mutants invaded the mansion, and picking a brief but violent fight with Wolverine. Hawkeye hunted down more of Madrox's dupes in the Savage Land before being ambushed by Sabretooth, an encounter he regretted surviving. He also attempted to assassinate Magneto with a piece of shrapnel, only for Quicksilver to sacrifice himself to save his father.
Following the Ultimatum Wave, Hawkeye offered to help Hank Pym search for his estranged wife Janet, revealing that his eyes had been mechanically enhanced (leading to his perfect aim) and believing that no one should have to endure the loss of a loved one. Unfortunately, Janet had been killed and was being eaten by the Blob when they found her. Hank Pym killed the Blob and took Janet's body back to the Triskelion, telling Hawkeye to activate the Jocasta Protocol before allowing himself to be blown up to stop a group of Madrox suicide bombers. Hawkeye then participated in an attack on Magneto himself.
The Red Skull
After the Ultimatum event, Hawkeye teamed with Captain America to stop the new A.I.M.group from stealing weapons from the Baxter Building, only to unexpectedly find Captain America being beaten down by his son, the Red Skull. Hawkeye later agreed to join Nick Fury's new Black OPS team, the Avengers, in order to stop Captain America, who turned rebel, from finding his son, and to also stop the Red Skull from unleashing extreme horror to the world. When Captain America was tracked down by S.H.I.E.L.D., and was discovered to be in France, the Avengers came in and tried to subdue Cap. Cap responded physically, and a fight ensued. Hawkeye tried to shoot Cap with tranquilizers, but Cap deflected the tranquilizers with his shield, shooting back into Hawkeye's neck. Later, Cap was apprehended.
The Ghost Rider
Clint would also participate in the hunt for John Blaze, the Ghost Rider. He is seen actively fighting Blaze at O'Hare Airport in Chicago, alongside the Punisher, as well as in Washington D.C., where the Avengers were charged with defending Vice President Blackthorne, who was revealed to be the cause of Blaze's death twenty years prior. After the fight had incapacitated both Tyrone Cash and War Machine, with the Punisher in pursuit of both the Vice President, who had turned into Vengeance with the help of Mephisto, Clint would follow as well.
There he learned the Punisher had let John Blaze go, while having killed the Vice President. He would beg Hawkeye to show the same mercy he had bestowed upon Blaze, only to be hit in the face. Later it is revealed that he had escaped, with Hawkeye having hit the implant hidden in Castle's molar, allowing him to escape without pursuit.
vs. The New Ultimates
Hawkeye is once again recruited by Nick Fury for an Avengers mission involving the recovery of stolen Super-Soldiers and S.H.I.E.L.D. secrets for sale on the Black Market. After taking out the Italian Drug Cartel he takes a flight home and joins the rest of the Avengers for the mission. During the sale of Maximus (a prototype Super-Soldier), the team encounters Tyrone Cash, who reveals that Carol Danvers is behind the sale and has been selling S.H.I.E.L.D. secrets for years. 
The Avengers confront the New Ultimates, which results in a massive battle in downtown New York. Hawkeye is trapped in a vehicle when Giant-Man grabs and compresses it with his hands. The Avengers are later arrested and taken aboard the U.S.S. Jimmy Carter. Hawkeye then escaped, along with the other Avengers, and they all ingested Cash's serum and gained temporary Hulk-like abilities. They rescued Fury, but were overpowered by Gregory Stark and tossed overboard. They later joined the New Ultimates in Korea against the Spider.
Return to the Ultimates
After the return of Nick Fury as director of S.H.I.E.L.D., Clint rejoined the Ultimates, using his old costume. Before rejoining the team, however, he spends days in the South East Asian Republic leading the Runaways and Bruce Banner/Hulk to claim a sample of the Source from the People, who are ruled by Xorn and Zorn. After the situation of the Children of Tomorrow, the President activated the Winter Protocols, allowing Agent Flumm to take control of S.H.I.E.L.D. and to seize the Ultimates, Clint escaped along Black Widow and Falcon, but where finally apprehended. He gathered with the rest of the Ultimates after Iron Man negotiated with the President for their liberty in exchange to help in the final defeat of the Children's leader, the Maker.