Since he was eighteen years old, Guerrero has been fighting in the Featherweight (126 lbs) division. Now twenty five, "The Ghost" feels he'll be physically more powerful at Jr. Lightweight (130 lbs) as he stated, "It's been my childhood dream to become a world champion and I've accomplished that goal at Featherweight. I'm truly appreciative to the IBF for giving me the opportunity to fight for their title. I've been at this weight since the amateurs. I've come to realize that I can no longer make Featherweight and feel strong. I'm older now, my body has matured, so its time for me to move up in weight. The extra four pounds will only make me stronger. At Jr. Lightweight I believe I'll be more effective."
Guerrero won his first world title on November 9, 2006 when he stopped Eric Aiken at the Staples Center due to a vicious body attack. Aiken could no longer continue after the eighth round, giving The Ghost a TKO vicory.
On February 23, 2007, "The Ghost" became two-time world champion when he traveled overseas and defeated home town hero, Spend Abazi, via ninth round TKO. Robert was put in the difficult situation to cross continents, when Orlando Salido, who out pointed him on November 4, 2006, tested positive with the powerful steroid nandrolone. Salido was stripped of the title giving Guerrero the opportunity to regain his championship belt. The bout was later ruled a no decision.
In his first title defense Guerrero knocked out Martin Honorio in the first 56 seconds of the opening round. The fight took place at the Desert Diamond Casino in Tucson AZ, on November 3, 2007.
February 29, 2008 at the Tachi Palace Casino in Lemoore CA, Robert knocked out Jason Litzau in the eighth round. A perfectly timed uppercut sent Litzu to the canvas, ending the contest at the 2:25 mark, as Guerrero defended his title for the second time.
"My goal now is to become a world champion at the Jr. Lightweight and Lightweight divisions." said Guerrero, "I want to challenge all the big names, guys like Manny Pacquiao and Juan Manuel Marquez. I feel confident in my ability to accomplish my new goals."