Men's 50m Freestyle
· This is the fastest medal event in all of competitive swimming (long course). The world record of 20.91, set by Cesar Cielo (BRA) in December 2009, equates to a land speed of 8.6 kilometres per hour.
· Cielo is a three-time world champion in this event, claiming the gold in 2009 in Rome, in 2011 in Shanghai and in 2013 in Barcelona.
· Two swimmers other than Cielo have won this event more than once at the world championships : Tom Jager (USA, 1986, 1991) and Aleksandr Popov (RUS, 1994, 2003).
· The defending champion from Kazan two years ago is Florent Manaudou, the only Frenchman to win the men's 50m freestyle at the world championships.
· Manaudou won that title as the reigning Olympic champion (from London 2012), the first to achieve this feat since Cielo in 2009 and 2011.
· The Olympic champion from Rio in 2016 is Anthony Ervin (USA), who won the Olympic medal at age 35. At 35 years and 78 days, Ervin became the oldest man ever to win an Olympic gold.
· Only one swimmer was older than Ervin when he took gold at Olympic Games or world championships. Mark Warnecke (GER) was 35 years and 162 days old, when he won the men's 50m breaststroke in Montreal in 2005.
· Ervin also won a world title in this event, in Fukuoka in 2001, when he was 20 years old.
· Ervin is one of four US swimmers who have won this event at the world championships. They have combined for five world titles, making United States the most successful country in this event at world championships.
· The United States has won 10 medals (G5-S4-B1) in this event at world championships, six more than the next most prolific countries.
· The last US swimmer to win this event at world championships was Ben Wildman-Tobriner (USA) in Melbourne in 2007.
· At all three Mare Nostrum meets in June 2017 (Monaco, Barcelona, Canet-en-Roussillon), the men's 50m freestyle was won by the same person: Bruno Fratus (BRA).
Women's 50m Butterfly
· Reigning women's 50m butterfly world champion, Sarah Sjöström (SWE), is hoping to become the second woman to win this title twice after Inge de Bruijn (NED, 2001-2003).
· Sjöström is the world record holder in this event (24.43) which she set at the Swedish championships in 2014.
· Sjöström can become the first woman to win both the 50m and 100m butterfly at least twice at the world championships.
· Lu Ying (CHN) can bag her third world championships medal in the 50m butterfly, after collecting silver in 2013 and bronze in 2015.
· Rikako Ikee (JPN) and Penny Oleksiak (CAN) finished respectively first and second place in this event at the 2015 junior world championships. The recently turned 17-year-olds can become the youngest women on the 50m butterfly podium at the world championships. This record now belongs to Danni Miatke (AUS) who won the title in 2005 at 17 years and 243 days.
Men's 100m Butterfly
· Chad Le Clos (RSA) has won the men's 100m butterfly at each of the last two world championships.
· Before Le Clos's double, Michael Phelps (USA) won this event at three consecutive world championships.
· Phelps is the only swimmer with three world titles in this event. Apart from Phelps and Le Clos, Ian Crocker (USA, 2) is the only swimmer who has won this more than once.
· Swimmers representing the United States have combined for nine of 16 available world titles in this event. Apart from the US, South Africa (2) is the only country to have won this more than once.
· At the Rio 2016 Olympic Games, Joseph Schooling (SGP) emerged victorious in a field that also produced a share of the silver by three different swimmers: Le Clos, Phelps and László Cseh (HUN).
· Schooling's was Singapore's first ever gold medal at the Olympic Games. It was also their first in another sport than weight lifting or table tennis.
· This made Schooling join Surinam's Anthony Nesty (SUR), who also won his country's only Olympic gold medal in the men's 100m butterfly.
· Nesty is one of the reigning Olympic champions that claimed the world title in this event at the next world championships (1991). After Nesty, Lars Frölander (SWE, 2001) and Phelps (2009) also succeeded at this.
· The United States has won 19 medals in this event at the world championships. The next most prolific country is Sweden on five.
· Hungary has won two medals in this event in this competition, both silver medals by László Cseh.
Women's 200m Backstroke
· Emily Seebohm (AUS) is the reigning world champion in the women's 200m backstroke. She could join Cornelia Sirch (GDR), Kirsty Coventry (ZIM) and Missy Franklin (USA) on a record two world titles in this event.
· Sirch (1982-1986) and Franklin (2011-2013) won the 200m backstroke world title at back-to-back world championships.
· Three-time medallist in this event and current world record holder, Missy Franklin, is out of competition due to injury.
· Maya DiRado (USA) won the gold medal in this event at the Rio 2016 Olympic Games. She has since retired from professional swimming.
· Katinka Hosszú (HUN) claimed the silver medal at the Olympic Games in Rio de Janeiro and finished third at the last world championships.
· Hosszú can become the second woman to grab a medal in the 200m backstroke in front of a home crowd after Lorenza Vigarani (ITA) who finished third in Rome in 1994.
· Only one Hungarian woman won the 200m backstroke world title, Krisztina Egerszegi (HUN) in 1991.
· Daria Kustinova (RUS) just missed out on a medal in Rio de Janeiro. She could join Anastasiia Fesikova (RUS, silver in 2009) and Stanislava Komarova (RUS, silver in 2001 and bronze in 2003) as the only Russian women with a world championships medal in the 200m backstroke.
Men's 4 x 200m Freestyle Relay
· In Kazan two years ago, Great Britain won this event for the first time ever at world championships.
· This ended a run of five successive world titles for the United States in the men's 4x200m freestyle relay, a run that started in 2005 and ended in 2013.
· The USA's world championship winning teams had steadily advanced in average, from 21 to 22 to 23 to 25, before entering the oldest team to ever win this event, at 26 years and 11 days in 2013.
· The British toppled the Americans with a team averaging 21 years of age. The team was anchored by 19-year-old James Guy (GBR), who also won the individual event, the men's 200m freestyle.
· It marked the first time in the history of the world championships that Great Britain had celebrated one of their teams winning a men's relay event at the world championships.
· The United States still leads the way with eight world titles in the men's 4x200m freestyle relay, five more than the next most prolific country in this event. Australia has won this on three occasions.
· Three swimmers aged 30 or older have won a medal in this event and two of the three achieved this in Kazan two years ago. Ryan Lochte (USA) won silver at 31 and Grant Hackett (AUS) won bronze at 35.
· Lochte, who is not participating in Budapest, has won a record six medals (G5-S1-B0) in this event, one more than Hackett (G3-S0-B2) and Michael Phelps (USA, G4-S1-B0).
· The six medals by Lochte in this event is joint-most by a male swimmer in a specific event at the world championships. Lochte himself has won six (G4-S1-B1) in the men's 200m individual medley and Fabien Gilot (FRA) has won six in the men's 4x100m freestyle relay (G2-S2-B2).
· The United States failed to win a medal in this event on three of 16 occasions it was staged (1975, 1994 and 1998). Their medal tally in this event is 13 (G8-S3-B2).
· At the Rio Olympic Games, USA won gold ahead of Great Britain and Japan.
· Japan's best result in this relay at the world championships was a fourth place in 2009.
Women's 800m Freestyle
· Katie Ledecky (USA) has won the women's 800m freestyle at the last two editions of the world championships.
· Ledecky can become the first woman to win this event three times at the world championships.
· The only women that claimed three medals in this discipline are Kate Ziegler (USA, G2-S0-B1), Lotte Friis (DEN, G1-S2-B0) and Diana Munz (USA, G0-S3-B0).
· Ledecky won this event at the Rio 2016 Olympic Games in a world record time (8:04.79), more than nine seconds faster than any other woman has ever clocked in this distance.
· The next best performer is Rebecca Adlington (GBR), who recorded 8:14.10 to take the gold medal at Beijing 2008.
· The 10 fastest times ever recorded in this event all belong to Ledecky.
· Ledecky won her first ever Olympic medal in the women's 800m freestyle when she won the gold medal at London 2012 at age 15.
· She can now become the first woman to win the 800m freestyle for the fifth consecutive time at the Olympic Games and world championships combined. Ledecky now shares the record of four with Janet Evans (USA) who won Olympic gold in 1988 and 1992 and the world title in 1991 and 1994.
· Boglárka Kapas (HUN) finished third place in the 800m freestyle at the Olympic Games in Rio de Janeiro. She could claim Hungary's first medal in this event at the world championships.
· Kapas can become the first woman to grab a world championships medal in this event in front of a home crowd since Alessia Filippi finished third in Rome in 2009.