After tonight, either Floyd Mayweather Jr. or Manny Pacquiao will be the welterweight champion of the world -- but both opponents in the boxing mega-match will be millions and millions of dollars richer, win or lose.
Mayweather may earn $180 million or more while Pacquiao's side will likely earn about $120 million, the Associated Press estimated.
The official weigh-in will take place on Friday night in Las Vegas before fight night begins on at 9 p.m. ET at the MGM Grand.
Mayweather, the highest-paid athlete in the world, according to Forbes, grew up in a boxing family.
His father and trainer, Floyd Mayweather Sr., fought and lost to the legendary boxing champ Sugar Ray Leonard in 1978.
Pacquiao, born on the southern coast of the Philippines, is spending his second term as a member of the House of Representatives there. As a teenager, he moved to Manila to earn money and soon started boxing.
One opponent of both Mayweather and Pacquiao, Oscar De La Hoya, described his struggle to predict a winner.
"My head goes with Mayweather, my heart is with Pacquiao," De La Hoya told ESPN. "I like Pacquiao. I like what he stands for, what he believes in. I like his fighting spirit. I respect the boxer [Mayweather] but I love the fighter [Pacquiao].
"We know what Mayweather is going to do," De La Hoya said. "He doesn't change his game plan. There's nothing new to his style. With Pacquiao, there is so much room for improvement that he might surprise us."
Floyd Mayweather, Jr.
Trainer: Floyd Mayweather, Sr.
Born: Grand Rapids, Michigan
Height: 5 feet 8 inches
Reach: 72 inches
Record: 47 fights, 47 wins, 26 KOs
Emmanuel Dapidran Pacquiao
Nickname: Manny, Pac Man
Trainer: Freddie Roach
Born: General Santos City, Philippines
Height: 5 feet 6 1/2 inches
Reach: 67 inches
Record: 64 fights, 57 wins, 38 KOs