Muhammad Ali, a boxing legend who won his first world heavyweight championship at 22 and later became one of the greatest athletes in U.S. history—died late Friday night, after being hospitalized with a respiratory illness in Arizona, where he lived out the final years of his life. Ali was 74. The cause of death is not yet known. 

NBC News first broke news of his death:

Ali, born Cassius Marcellus Clay Jr., grew up in Louisville, Ky. He started training to become a boxer at 12, after a policeman who ran a local recreation center encouraged him to learn how to fight in order to seek revenge on a thief who had stolen his bike. Ali participated in his first professional fight in 1960, and achieved 19 consecutive victories—including 15 by knockout—before upsetting Sonny Liston to win his first heavyweight title during his twentieth fight in 1964.

It was before his fight with Liston that Ali famously promised to “float like a butterfly and sting like a bee." He also taunted Liston by telling him, “Your hands can’t hit what your eyes can’t see.” Many boxing analysts have since used those phrases to describe Ali's unorthodox boxing style, which involved relying on hand speed and movement in the ring, rather than brute strength. The phrases also established Ali as one of the premier trash-talkers in boxing.

Shortly after his victory over Liston, Ali joined the Nation of Islam (NOI), whose leader Elijah Muhammad announced that Ali's birth name (Cassius Marcellus Clay Jr.) would be changed to Muhammad Ali.

Ali continued his boxing career, and recorded nine more victories—including a rematch win against Liston—before he was suspended in 1967 after getting arrested for refusing to be drafted into the U.S. Armed Forces. Ali cited his religious beliefs as the reason behind his refusal, and spoke out about his disdain for America’s role in the Vietnam War. During this period, Ali established himself as an important figure outside the boxing ring, as he often discussed racial inequality and religious freedom in the context of his anti-war views.

“Why should they ask me to put on a uniform, and go 10,000 miles from home and drop bombs and bullets on brown people in Vietnam, while so-called Negro people in Louisville are treated like dogs and denied simple human rights?” Ali once asked.

Although he was found guilty of draft evasion in 1967, Ali filed an appeal, and eventually took his fight all the way to the U.S. Supreme Court where his conviction was overturned. The court's ruling allowed Ali to get back into the ring in 1971, after which he enjoyed some of the most memorable fights of his career. Ali, who eventually earned the nickname “The Greatest,” lost his first professional fight to Joe Frazier in March 1971, which was later dubbed “The Fight of the Century.”

However, he rebounded from his loss to Frazier to win 10 consecutive fights, before losing to Ken Norton in March 1973; he then defeated Norton in a rematch in September 1973.

Ali also bounced back from his loss to Frazier by beating him in January 1974 during “Ali-Frazier II.”

Ali’s next fight in October 1974 saw him defeat George Foreman in the infamous “Rumble in the Jungle” match.

Ali also defeated Frazier for a second time in October 1975 during the legendary “Thrilla in Manila” match.

Ali continued boxing through December 1981, when he lost to Trevor Berbick in his 61st and final professional fight. He then retired from boxing with a 56-5 record, and achieved 37 knockouts during his career.

Less than three years after he retired from boxing, Ali was diagnosed with Parkinson’s disease at the age of 42. Doctors told him his Parkinson’s was likely the result of the punches he took to his head throughout his career. Despite his diagnosis, Ali continued to make public appearances across the U.S. and world. He traveled to Iraq in 1991, and spoke with Saddam Hussein about releasing American hostages at the height of the Gulf War. He lit the flame at the 1996 Summer Olympics, even though Parkinson’s made it impossible for him to do it without shaking. He even helped promote his 2001 biopic, Ali, starring Will Smith.

In recent years, health concerns have limited the number of Ali's public appearances, but he's continued to raise funds for the research and treatment of Parkinson’s. He helped open the Muhammad Ali Parkinson Center in Phoenix in 2009, and he regularly worked with an organization called Celebrity Fight Night to fund the center. Ali’s last official public appearance came in October 2015 at the Sports Illustrated Tribute to Muhammad Ali at the Muhammad Ali Center in Louisville.

Ali is survived by his wife, Yolanda “Lonnie” Williams, his two sons, Asaad Amin and Muhammad Jr., and his seven daughters, Laila, Rasheda, Hana, Maryum, Jamillah, Khaliah, and Miya.