On 16 September 2007, two days after West Coast's semi-final elimination by Collingwood, it was announced that Judd had left West Coast and would be requesting a trade to a club in Victoria. He notified West Coast coach John Worsfold and CEO Trevor Nisbett of his intentions earlier that day. As arguably the most talented player in the competition, his departure created much attention and speculation among the Melbourne-based clubs, the media and the football community.
In the weeks following the announcement of his departure, Judd met with four clubs: Essendon, Melbourne, Collingwood and Carlton. On 2 October 2007, Judd announced that his preferred club was Carlton, and Carlton was also considered most likely to secure a trade with West Coast, because the club held two early draft picks which could be used in negotiations. On 11 October 2007, Judd was officially traded to Carlton along with a third round selection in the 2007 AFL Draft (No. 46 overall) for Carlton's first and second round selections (No. 3 and 20) and Josh Kennedy, who was reluctant to leave Carlton. Judd was subsequently given the No. 5 guernsey vacated by the trade of Kennedy, and he signed a six-year, $6,000,000 contract with the club.
At his peak, Judd was the best midfielder in the game and is widely regarded as one of the best players of the 2000s decade. The strengths and traits of Judd's playing style were different between his time at West Coast and his time at Carlton, and he has been widely lauded for his proficiency at both. At West Coast, while he was a strong ball-winner, it was as an outside midfielder and ball-user where he distinguished himself. He possessed a combination of explosive speed, acceleration, agility and core strength which few if any players in the league could match; these attributes gave him the ability to receive the ball in traffic, then break free from or weave around taggers and opponents, allowing him to take clearing kicks in open space which were damaging to opposition teams. By the time he had joined Carlton, he had lost much of his acceleration and agility owing to the groin injuries he suffered in 2007. He overcame this by converting his game style to predominantly inside ball-winning role. By virtue of his core strength and balance, he became one of the best in the league at receiving ruck tap-outs, and riding or shaking off tackles in packs and congestion to win clearing handpasses to Carlton's outside midfielders.