A hyperactive child, Phelps was encouraged into swimming aged seven to give his boundless energy an outlet, and became the most decorated Olympian of all time.
The 'Baltimore Bullet' came home empty-handed from his first Games at Sydney 2000 when just 15.
But a slew of world records over the next three years led to a dominant display at Athens 2004 as Phelps took six gold and two bronze medals, the second-best performance at an Olympic Games after fellow US swimmer Mark Spitz's seven golds in 1972.
At Beijing four years later, Phelps claimed the all-time record when in the 4x100m medley relay he completed a haul of eight golds in one Games - seven of them with world record times.
Australian arch-rival Ian Thorpe had prior to Athens said it would be "impossible" to win eight golds - a statement which Phelps kept on his locker as a motivation.
At London four years later he became the most decorated Olympian of all time, taking his total to 18 golds, two silvers and two bronzes.
Having just turned 31, he is aiming to extend his record in his fifth Games this year, having qualified for three individual events at the recent US trials by winning the 100m butterfly, 200m butterfly and 200m individual medley.