Eight players are selected for the Home Run Derby and compete in a traditional playoff system, in which the players with the most home runs advance to the next round. Each player gets ten "outs" per round; in this case, an out is defined as any swing that is not a home run. Should a tie exist between players at the end of any round, there will be a five swing swing-off to determine who will advance, followed by extra swings if needed. Until 2006, the home run count was reset after each round. However, a rule change was made for the 2006 Home Run Derby which causes the home run count for the four players advancing to the second round to carry over. The home run count for the final round is still reset to zero.
Prior to 1991, the Home Run Derby was structured as a two-inning event with each player receiving five outs per inning which allowed for the possibility of ties.
In 2000, a match play format was instituted for the second round. The player with the most home runs in the first round faced the player with the least among the four qualifying players, as did the players with the second and third-most totals. The contestant who won each matchup advanced to the finals. This format was discontinued after the 2003 Derby.
For the most part, the field of players selected consists of four American League players and four National League players. The first Derby in 1985 featured five from each league, and the Derbies in 1986 and 1987 each featured three and two players from each league, respectively. In 1996, the field was again expanded to ten players, five from each league, though in 1997, the American League had six contestants to the NL's four. In 2000, the field reverted to the four-player-per-league format, which it has remained ever since. The only exception has been in 2005, when Major League Baseball chose to change the look of the contest, still having eight players, but with the players representing their home countries rather than their respective leagues. This was commonly looked upon as a lead-in to the World Baseball Classic which was played in March 2006. Bobby Abreu, representing Venezuela, won the first International Derby with a record 41 home runs, including a then-record 24 in the first round, broken only by Josh Hamilton with 28 home runs in 2008. In 2006, the selection of four players from each league to participate in the Derby was resumed. In 2011, the format was revised so that there were team captains selecting the sides. Leading the American League will be David Ortiz of the Boston Red Sox, while Prince Fielder of the Milwaukee Brewers will lead the National League.
Since 2005, a gold ball has been used once a player reaches nine outs. If a batter hit a home run using the golden ball, Century 21 Real Estate and Major League Baseball would donate $21,000 (a reference to the "21" in "Century 21") per home run to charity (MLB donated to the Boys and Girls Clubs of America, and Century 21 donated to Easter Seals). In both 2005 and 2006, $294,000 was raised for the charities, equaling fourteen golden ball home runs per year. State Farm continued this in 2007 as they designated $17,000 per home run (one dollar for each of State Farm's agencies), to the Boys and Girls Clubs of America. In the 2007 event, fifteen golden balls were hit for a donation of $255,000, and ten ($170,000) were hit in the 2008 event. For 2009, State Farm added $5,000 for all non-Gold Ball homers, and $517,000 was collected. For 2010, the non-Gold Ball homer was reduced to $3,000 per home run and a total of $453,000 was collected.