Muhammad Ali (born Cassius Marcellus Clay, Jr.; January 17, 1942) is an American former professional boxer, philanthropist and social activist. Considered a cultural icon, Ali was both idolized and vilified.

Originally known as Cassius Clay, Ali changed his name after joining the Nation of Islam in 1964, subsequently converting to Sunni Islam in 1975, and more recently practicing Sufism. In 1967, three years after Ali had won the World Heavyweight Championship, he was publicly vilified for his refusal to be conscripted into the U.S. military, based on his religious beliefs and opposition to the Vietnam War. Ali stated, "I ain't got no quarrel with them Viet Cong... No Viet Cong ever called me nigger"–one of the more telling remarks of the era.

Widespread protests against the Vietnam War had not yet begun, but with that one phrase, Ali articulated the reason to oppose the war for a generation of young Americans, and his words served as a touchstone for the racial and antiwar upheavals that would rock the '60s. Ali's example inspired Martin Luther King Jr.–who had been reluctant to alienate the Johnson Administration and its support of the civil rights agenda–to voice his own opposition to the war for the first time.

Ali would eventually be arrested and found guilty on draft evasion charges; he was stripped of his boxing title, and his boxing license was suspended. He was not imprisoned, but did not fight again for nearly four years while his appeal worked its way up to the U.S. Supreme Court, where it was eventually successful.

Ali would go on to become the first and only three-time lineal World Heavyweight Champion.

Nicknamed "The Greatest," Ali was involved in several historic boxing matches. Notable among these were three with rival Joe Frazier, which are considered among the greatest in boxing history, and one with George Foreman, where he finally regained his stripped titles seven years later. Ali was well known for his unorthodox fighting style, which he described as "float like a butterfly, sting like a bee", and employing techniques such as the Ali Shuffle and the rope-a-dope. Ali had brought beauty and grace to the most uncompromising of sports and through the wonderful excesses of skill and character, he had become the most famous athlete in the world. He was also known for his pre-match hype, where he would "trash talk" opponents, often with rhymes.

In 1999, Ali was crowned "Sportsman of the Century" by Sports Illustrated and "Sports Personality of the Century" by the BBC.