The Derby, part of Gatorade All-Star Workout Day, will be broadcast live on ESPN and MLB.com beginning at 8 p.m. ET tonight.
MLB.com's coverage will include a live feed of the Derby, a Twitter feed and real-time results.
Pena joins Tigers third baseman Brandon Inge, Rangers outfielder Nelson Cruz and Twins catcher Joe Mauer from the Junior Circuit, and the National League features big league homer leader, hometown hero and All-Star centerpiece Albert Pujols of the Cardinals, plus Adrian Gonzalez of the Padres, Brewers first baseman Prince Fielder and Ryan Howard of the Phillies.
"I thought that it would be only fitting for me to experience the whole entire thing," Pena said of being named to the All-Star team and the Derby on the same day.
"This is a huge deal to me. I'm not going to even pretend to play it cool or professional. I'm in shock. I've called half the world."
Pujols is naturally the choice as the heavy favorite in this competition.
Not only will Pujols be swinging for fences that he knows -- and clears -- better than anyone in the game, but he's also on the way to the biggest power year in his otherworldly career. Pujols had 32 homers entering Sunday's games -- eight more than Gonzalez, the NL runner-up, or Pena, the AL leader.
Pujols will also participate in the "Call Your Shot" promotion, in which a fan points to a spot in the stands and Pujols attempts to hit a batting-practice home run to that area. If he is successful, the fan wins a prize package.
"It's good -- it's a great honor to be an All-Star and have the opportunity to be in the Home Run Derby in your hometown where you play," Pujols said. "Hopefully, I can perform good for the fans, and I'm pretty excited."
Pujols' two past Derby performances indicate flashes of brilliance. In 2003 at U.S. Cellular Field, he hit a Derby-high 14 homers in the semifinal round but lost, 9-8, in the final to then-Angels outfielder Garret Anderson.
In his only other Derby appearance, in 2007 at San Francisco, Pujols worked overtime. First, he won a swing-off against the Twins' Justin Morneau to get out of the first round. Then he hit nine homers in the semifinals, but because eventual winner Vladimir Guerrero matched Pujols with nine, Guerrero -- who had hit five first-round homers to Pujols' four -- advanced based on total numbers.
This year, Pujols said he's just happy to be celebrating the Midsummer Classic in St. Louis along with Cardinals catcher Yadier Molina and closer Ryan Franklin.
"I thank the fans for voting, not just for me, but for my teammates, too," Pujols said. "To have the opportunity to be in such a great event with some of the other great players in the game, obviously, I'm really humbled and honored to be there."
Howard is honored, too, and he's the only contestant in this year's Derby to have won this event before.
Howard -- a native of nearby Wildwood, Mo., who attended Missouri State University -- accomplished the feat in 2006 at PNC Park in Pittsburgh, beating David Wright of the New York Mets, 5-4, in the final round. This year, Howard is bringing a secret weapon in the form of Deron Spink, his high school summer baseball coach, who will be Howard's personal pitcher.
"He knows me as well as anybody, as far as hitting goes," Howard said. "So I felt kind of like it was the place where it all began, and I just wanted to ask him, because I knew he would get a kick out of it."
Fielder is the only other Derby participant in either league who has tried this event in the past. The Brewers slugger didn't make it out of the first round in 2007 at San Francisco, hitting three homers. But he said he'll enjoy trying to do better this time.
"The fans like it," Fielder said. "I know the All-Star Game is 'for real' and everything, but I feel like it's more for the fans. If they want to see me, I'll be there."
Five first-timers will be there, too.
Gonzalez is the first Padres player to compete in the Derby since Gary Sheffield and Fred McGriff participated in 1992, when the All-Star Game was played in San Diego.
"It's exciting, and it's going to be fun," Gonzalez said. "There's always some fear of not hitting any home runs, although I wouldn't be the first or the last to do that. But hopefully, I can get into a good streak and hit a few of them."
Inge, who earned his All-Star spot by beating out four other players in the 2009 All-Star Game Sprint Final Vote, said the Derby might be his one shot at All-Star glory.
"Let's face it -- I was the last American League [player] picked to be in the game, so who knows if I will even get a chance to play?" Inge said. "But I know for a fact that I am going to [participate] in the Home Run Derby, and that really is going to be a lot of fun."
Cruz, meanwhile, has the unique responsibility of carrying the Rangers' torch into this year's Derby after his teammate, Josh Hamilton, put on perhaps the most memorable show in the history of the event last year, blasting 28 balls out of Yankee Stadium in the first round alone.
"I think I have a chance," Cruz said. "I don't expect to hit 20 homers like Josh."
And then there's Mauer, who leads the AL in hitting, which, as a two-time batting champion, is normal. But through Saturday, he also had 15 homers in 236 at-bats this year when his previous best power output was 13 long balls in 521 at-bats in 2006.
So Mauer has that going for him, which is nice, and he's got his high school coach, Jim O'Neill -- from Cretin-Derham Hall in St. Paul, Minn. -- heading to the Gateway City to pitch to him.
"I wanted to make sure that he could do it, and here we are," Mauer said. "I just think it's a great event. I always told my buddies when we were younger -- we always played it in the backyard and stuff like that -- I always told them, 'If I ever had the opportunity to do it, I would jump at it,' and it happened this year."
In addition to the regular festivities of eight Major League sluggers trying their hardest to hit ball after ball out of Busch Stadium, the Derby will feature other wrinkles.
Once again, eight local Boys & Girls Club members will each be paired with a Derby participant to win a donation toward a new teen center. The club member paired with the winner gets a $50,000 donation for his or her Boys & Girls Clubs, while the remaining children each will receive a $10,000 donation for their respective clubs. Also, four local Boys & Girls Clubs members will serve as Derby shaggers.
The Derby also includes the "Gold Ball" charitable component, instituted in 2005, in which Rawlings, the official supplier of baseballs to Major League Baseball, has created a specially designed gold baseball that will be used in every round when a player is down to his final out. For every home run hit by a Derby participant after his ninth out, Major League Baseball and State Farm will combine to donate $17,000 (representing State Farm's 17,000 agents) to Boys & Girls Clubs of America.
Also, MLB and MasterCard Worldwide announced recently that Stand Up To Cancer, for the first time, will be the recipient of donations in the "Hit It Here" in-stadium promotion. The first home run to directly strike one of the MasterCard "Hit It Here" signs in Busch Stadium during the Home Run Derby or the 2009 MLB All-Star Game will result in MasterCard donating $1 million to Stand Up To Cancer.
With so much going on, grateful newcomer Pena said he'll just try to focus -- by not focusing.
"The last three years, in batting practice, I've totally just forgotten about hitting home runs in batting practice," Pena said. "And the ones that I do hit I try to hit them the other way. So it's something that I totally haven't practiced in years, to actually pull a ball for a home run.
"But who cares? I just want to be in it."