He began his professional playing career with Blackburn Rovers, with whom he won the Football League Cup, and in 2003 was signed for £17 million by Chelsea, where he won two Premier League titles and another League Cup. After four seasons at Stamford Bridge, he left for Newcastle United where he won the 2006 UEFA Intertoto Cup, and then moved on to Fulham, with whom he played in the 2010 UEFA Europa League Final. He ended his career with brief spells in the A-League with Melbourne City and the League of Ireland with Shamrock Rovers before retiring in December 2015.