Michael Phelps was 15 years old in the 2000 Olympics. He finished fifth in the 200-meter butterfly, his only event of the games.
Today, the "Thorpedo" does occasional media commentary, owns an aquatic center in Australia, and recently launched a line of pool cleaners called Thorpedo.
Nicknamed "Eric the Eel," Eric Moussambani, a swimmer from Equatorial Guinea, became a fan-favorite for his inspirational story, though he finished the 100-meter freestyle with the worst time in Olympic history.
Moussambani will attend the Rio Olympics as the coach of the Equatorial Guinea swim team.
Source: Olympics
Misty Hyman became the hero of US Swimming when she pulled off a major upset over Australia's Susie O'Neill to win gold in the 200-meter butterfly.
Hyman didn't qualify for the 2004 Olympics and retired from swimming. Today, she's an assistant coach for Arizona State University's swim team.
Source: mistyhyman.com
Dara Torres was another US swimming star, winning five medals, two gold in relays, after taking a seven-year break from swimming from 1992 to 1999.
Torres won 12 medals in her Olympic career. She is now an author and CBS Sports Network panelist.
Source: Twitter/Dana Torres
Russian gymnast Alexei Nemov won the most medals in the entire games, pulling in two individual golds.
It's unclear what Nemov currently does, though he made headlines in 2016 for getting in an altercation with a political activist.
Source: Daily Mail
Widely considered the greatest 400-meter runner in history, Johnson now works with BBC and owns a training company called Michael Johnson Performance. He will be with BBC at Rio 2016.
Maurice Greene also starred for US track, winning a gold in the 100 meters and the 4x100-meter relay.
Greene retired as one of the best male sprinters, and has since appeared on "Dancing With the Stars" and volunteered as a track coach at UCLA.
Jones was later stripped of those medals after admitting to taking performance-enhancing drugs. She has since written a book and become a public speaker.
Source: marionjones.org
Sprinter Cathy Freeman was a star for Australia in 2000. An Indigenous Australian, she won gold in the 400 meters and carried the torch in a spectacular opening ceremony.
Today, Freeman runs her own foundation focusing on the education of indigenous children and is an IOC Sport and Active Society Commission Member
Vince Carter was one of the stars of the US basketball team after pulling off "le dunk de la mort" — his wild dunk over 7-foot French center Frederic Weis.
Swoopes was inducted into the Hall of Fame in 2016 and was coaching the Loyola Chicago women's basketball team but was fired in 2016.
American wrestler Rulon Gardner became a legend when he pulled off a massive upset over Russian wrestler Alexander Karelin, who had not lost in 15 years.
Gardner has had a bit of a wild journey since 2000. He's attempted several comebacks, struggled with weight, appeared on "The Biggest Loser," and worked briefly as an analyst. He attempted to make the trials for Rio 2016 but was unsuccessful.
In 2000, Venus, 20, and Serena Williams, 18, won gold in women's doubles. Venus also won gold in women's singles.
Now, Serena is 34, Venus is 36, they're ranked first and sixth in the world, respectively, and own 114 combined singles titles.
In 2000, Lance Armstrong was 28 years old and fresh off his first Tour de France victory. He won bronze in the individual time trial.
Armstrong, of course, after his immense doping scandal, was stripped of his Olympic medal. Now, Armstrong has various projects, including a podcast.
Source: lancearmstrong.com