There were 15 tournaments that provided qualifying opportunities to players. Most tournaments offered a qualifying position for the winner and runner-up of the tournament, however the World Championships and the 2007 Grand Slam also offered a place in the tournament to the losing semi-finalists. There were also various other ways of qualifying for overseas players, including those from Australia and the United States of America, as well as a wildcard qualifying event open to any darts player.
Note: Players in italics have already qualified for the tournament.
For the second year running, Martin Adams declined his invitation to take part in the competition.
Due to the 2008 World Masters being scheduled for December, Michael van Gerwen was offered an invitation to compete in the 2008 Grand Slam of Darts. The invitation was given as van Gerwen won the World Masters in 2006.
The PDC North American Order of Merit ranks all of the North American dart players based on the amount of prize money that they have won from tournaments held in the United States and Canada.
Having won the inaugural North American Darts Championship and reaching the second round of the 2008 Las Vegas Desert Classic, as well as reaching the later stages of some North American events on the PDC Pro Tour, Darin Young picked up approximately £9,075 in PDC tournaments in North America, meaning he topped the North American Order of Merit and was selected for inclusion the Grand Slam.
The William Cross Pro AM was a group of four darts tournaments held in Australia from the 2 to 10 August 2008. At the culmination of the four tournaments, an invitation to the 2008 Grand Slam of Darts was awarded to the Australian/New Zealand player who had accumulated the highest amount of prize money.
UK born player Paul Nicholson performed well in all four events and was runner-up in the Australian Open Players Championship. He topped the prize money table and as such was awarded a place in the 2008 Grand Slam of Darts.
The PDC also invited the winner of the BDO Women's World Championship this year. The winner of the 2008 event was the Russian Anastasia Dobromyslova, who defeated Trina Gulliver in the final. She was entered as a reserve player should the field remain unfulfilled, and with Phil Taylor, Raymond van Barneveld, and James Wade dominating most televised finals, she was invited to the event.
On 1 August 2008, the PDC announced that the ITV Wildcard qualifier was to be held on Friday 17 October 2008 at the Magnum Centre in Irvine, Scotland. Any darts player was able to enter and the winner received an invitation to the 2008 Grand Slam of Darts. The losing semi-finalists also competed in a third-place play-off match to determine reserve qualifiers. Together with the runner-up, these qualifiers would be invited to the Grand Slam of Darts should the 32-man field remain unfilled.
Newton, McGarry, and Tabern all qualified though Caven was the unlucky player to miss out – he was provisionally in the tournament until Adrian Lewis reached the final of the 2008 European Championship, the last qualifying event before the Grand Slam.
As a curtain-raiser for the main Grand Slam of Darts, a competition involving eight celebrities and eight of the professionals taking part in the Grand Slam took place on the Friday before the tournament. The winners of the tournament collected a first prize of £8,000. In a closely fought final, James Wade and Steve Backley won a final-leg decider over Phil Taylor and Neil Ruddock, with Backley's double two checkout sealing the victory. Those involved were:
With Andy Hamilton and Alan Tabern finishing level on points and leg difference, a piece of history was made, with a nine-dart shootout between the two to see who would play Phil Taylor in the second round. The match took place after the conclusion of Tuesday's second round matches.
In the seventh second round match, James Wade recorded his first televised nine dart finish against Gary Anderson, during the twelfth leg of Anderson's 10–8 win. Wade hit consecutive maximums, before checking out with treble 20, treble 19 and double 12. It was the first ever nine-darter at the Grand Slam and the first UK televised nine-dart finish in which the player achieving it eventually lost the match. In 2007 Michael van Gerwen recorded a nine dart finish during the semi-finals of the Masters of Darts (a tournament that was shown on Dutch TV) but ended up losing the match to Raymond van Barneveld.
Five days after the tournament concluded, the PDC revealed that the organisation had received complaints concerning the Group H match between Darryl Fitton and Gary Anderson, and forwarded those complaints to the Darts Regulation Authority. On 26 January 2009 it was announced that there was no evidence of player collusion in the arranging of match outcomes.