The greatest Olympian of all time came out of retirement and is now the first male swimmer to make five Olympics. Now a father (and sober), Phelps is looking to add to his 18 gold and 22 overall medals with a smaller docket. He's only competing in three individual events -- the 200 individual medley, the 100 butterfly and the 200 butterfly (he'll be on at least one relay) -- but can win an unprecedented fourth straight gold medal in the former two.
Bolt's Olympics appeared to be over before they started, after he injured his hamstring during the Jamaican Olympic trials, but he made the team on a medical exemption. The world's fastest man will attempt to complete the first-ever "triple triple" in Rio: win three straight golds in the 100-meter and 200-meter sprints, and the 4x100-meter relay.
The reigning 200-meter Olympic champion was attempting to be the first woman to complete the 200-400 double (Michael Johnson did it in 1996), and the International Olympic Committee even adjusted the track and field schedule to accommodate her. But, hampered by an ankle injury she suffered in April, her goal was dashed when she failed to make the team in the 200 -- her signature event -- by a heartbreaking one-hundredth of a second. She's still got the 400 meters, the 4x100-meter relay and the 4x400-meter relay.
The American women have won two team gymnastics gold medals, but this is the first time that they are expected to obliterate the competition. That's because of Simone Biles, the undisputed best gymnast in the world right now, who is the three-time defending all-around world champion. Two members of London's Fab Five, Aly Raisman and Gabby Douglas, are also back, and are the first U.S. gymnasts to make two straight Games since 2000. Douglas is the first all-around Olympic champion to return to the next Games since Nadia Comaneci in 1980. Meanwhile, 16-year-old dynamo Laurie Hernandez finished second behind Biles at trials, and Madison Kocian is one of four reigning uneven bars world champions.
How dominant is the long-distance swimmer? Since she captured gold as a 15-year-old in the 800-meter freestyle in London, Ledecky has never finished out of first place in international competition. She's 15 for 15, with nine world titles and five Pan Pacific Championships titles. Ledecky, who will also swim the 200 and 400 in Rio, owns the top 11 fastest times in history in the 800.
Fraser-Pryce has a chance to enter the history books before her countryman Usain Bolt just because of the scheduling. The two-time defending champion in the 100 meters could become the first Olympian to win three consecutive golds in the dash, as the women's final is a day before the men's. She's not running the 200, in which she's the defending silver medalist, in order to concentrate on the 100.
After finally completing his career Grand Slam at the French Open in June, the top-ranked tennis star will try to become just the third male player, after Andre Agassi and Rafael Nadal, to achieve the career Golden Slam (win all four majors and Olympic gold). He won bronze in 2008 and lost the bronze medal match in 2012.
The swimmer has posted the fastest time in the 100-meter freestyle this year and is the favorite in the event, just like his compatriot James Magnussen was in 2012. McEvoy is hoping to avoid The Missile's fate, when he was nipped by Nathan Adrian by one-hundredth of a second for gold in London. Adrian, who was the first American to win the 100 free since 1988, will be back in Rio.
The 2012 decathlon champion is the overwhelming favorite to defend his crown. He has not lost a major title since then, and set a world record of 9045 points at the world championships last year.
In 2012, the pair became the first Aussie sisters to compete at the Olympics in 40 years, with Cate winning gold in the 4x100-meter freestyle relay and Bronte finishing 10th in the 50-meter freestyle. They shared a podium for the first time at the 2015 World Championships, when Bronte won gold in the 100 free and Cate took bronze, and could be the first sisters to share a podium at the Olympics.
The 16-year-old became the youngest American track and field start to qualify for the Olympics in 40 years, when she finished third in the 400-meter hurdles at trials. She turns 17 on Aug. 7, five days before track and field events start in Rio.
In a sport where you peak in your teens, the gymnast is heading to her seventh Olympics at the age of 41. Chusovitina won team gold in 1992, when she competed for the Unified Team, and a silver in vault in 2008, when she competed for Germany. The vault is her best chance for a medal in Rio.
The fencer will be the first Muslim-American woman to compete in a hajib and hopes to be a unifying symbol to counter the fear-mongering rhetoric about Muslims. Ranked second in the U.S. and eighth in the world, Muhammad has won five world championship medals, including one gold.
Nicknamed Superman, the reigning all-around Olympic champion and six-time world champion is the undisputed top dog of men's gymnastics. He's aiming to be the first back-to-back Olympic all-around winner since his countryman Sawao Kato did it in 1968 and '72.
Ennis-Hill won gold in the heptathlon at her home Olympics four years ago and is looking to be the first woman to defend her title since Jackie Joyner-Kersee in 1992. After giving birth to her first child in 2014, Ennis-Hill bounced back to win her second world title in 2015.
Walsh Jennings is gunning for her fourth straight beach volleyball gold - but it would be her first without longtime partner Misty May-Treanor, who retired after London. Walsh Jennings and Ross, who won silver in London with former partner Jennifer Kessy, barely made the cut for Rio, because Walsh Jennings was sidelined until March after her fifth shoulder surgery.
The Splash judge upset China's Qiu Bo in the 10-meter platform in London, but Qiu has won the two world titles since over Boudia, setting up quite the juicy rematch in Rio. Boudia would become the first back-to-back Olympic champ in the event since Greg Louganis in 1988.
As a 17-year-old, Shields won the first-ever women's middleweight gold in London. She has not lost a bout since, taking home two world titles and the Pan American title in the past three years.