Bolt was already an accomplished 200-meter runner before Beijing, but his record-setting 100-meter victory in 2008 made him a household name.
The first man to win six golds in Olympic sprinting was named the IAAF's World Athlete of the Year five times in six years, from 2008 through '13.
After becoming the first modern Olympian to win back-to-back 100- and 200-meter golds in 2012, Bolt has a chance to break a tie with Carl Lewis for most world and Olympic gold medals.
Phelps became the youngest U.S. Olympic swimmer in 68 years when he debuted in 2000 at 15 years old, finishing fifth in the 200-meter butterfly.
Since 2000, he has won 77 Olympic, world championship and Pan-Pac medals. His 22 total medals and 18 Olympic golds are records.
Coming out of retirement in 2014—after a DUI arrest and a rehab stint—he is looking to become the oldest swimmer, 31, to win an individual gold, and the first to win individual golds 12 years apart apart.
As a 19-year-old in his Olympic debut, Uchimura took silver in the all-around in Beijing, Japan's first medal in the event since Koji Gushiken's gold in 1984.
The first gymnast to win every major all-around title between Olympiads, he won six straight world all-around competitions from 2009 to '15.
In addition to becoming the first man since Japan's Sawao Kato in 1972 to win successive all-around golds, Uchimura hopes to help Japan avenge two straight second-place finishes to China.