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Since first winning 200m butterfly gold at the world championships in Fukuoka in Japan in 2001, Phelps added 25 gold medals, six silver and one bronze - and that from the worlds alone.
In between world championships, he became the most successful Olympian by a long shot as he won 18 golds, two silver and two bronze between 2004 and 2012.
However, the American superstar retired at age 27 at the end of the London Olympics and the 14th FINA world championships, which are being held from July 19-August 4 in Barcelona, is the first major meet since 2000 in which Phelps will not participate.
Even without Phelps, the United States are sending a very strong team to Barcelona and are the favourites to top the medal standings in the swimming competition which starts on Sunday.
In Missy Franklin and Ryan Lochte, the US team has two potential stars of the championships. The teenager Franklin is going for eight golds (five individual and the three relays), hoping to become the first swimmer to win eight titles at a single world championships.
US women's coach Dave Salo told journalists earlier in the week that he was confident Franklin would manage her mammoth task. "It is a big load, but she had a big preparation for that in London, and I think she's able to handle it."
The coach will rest the 18-year-old in the relay heats. "We have people to help on the relay prelims, so that will give her a chance to rest a bit."
Another swimmer who could make history is 15-year-old Katie Ledecky, who is the first American swimmer to qualify for the 200m, 400m, 800m and 1 500m freestyle events.
Meanwhile Lochte, who won five worlds golds in Shanghai two years ago, has entered six events and showed in the US trials that he was nearing his best again after taking time out to star in his own reality TV show after the Olympics.
The US, of course, are not the only team with strong swimmers, with China hoping to continue their inroads towards the traditionally leading swimming nations, US and Australia.
Sun Yang, who smashed Grant Hackett's 1 500m mark in Shanghai and then bettered it again in London, will compete in the 400m, 800m and 1 500m, while teenager Ye Shiwen will be out to prove that her sensational win in the 400m medley was above board.
The 17-year-old, who had to dodge a barrage of doping questions in London, said that she has put that aside.
"I was very depressed and angry after London but everything is fine now. It is a long journey. Different people have their different views but I will just keep on going," she told English media.
Also in the spotlight will be 16-year-old Ruta Meilutyte of Lithuania, who stunned the world with a win in the 100m breaststroke in London.
South Africans, meanwhile, are hoping for another stand-out performance from Chad le Clos, who beat Phelps in the 200m butterfly in London. The 21-year-old will swim in the three butterfly events in Barcelona and said beating Phelps had meant so much to him.
"I am now going into a world championships as the man having a target on his back and that is different for me but I just have to prepare for the race as I usually do.
"My whole career was about, with respect, trying to beat Michael Phelps. I always wanted to race the best in world," he said in a press conference in Barcelona.
Le Chad, who said he would be going shark cage diving with Phelps in South Africa, said he had no idea whether the American was planning a come-back.
Certainly though, Phelps will not be swimming in Barcelona.
After the world record breaking - albeit assisted by hightech swim suits - worlds in Rome in 2009, many said that the world records will stand for years to come.
That has proven not to be the case and records were broken at the worlds in Shanghai and at the Olympics in London last year.
It seems likely that those who believe that the world championships have lost their appeal without Phelps will be similarly proven wrong.
