From Wikipedia: Hartley signed for Celtic for £1.1 million on a two-and-a-half-year contract, with the option for a further year, in January 2007. Despite having a mediocre start to his Celtic career in the second half of the 2006–07 season, Hartley enjoyed a very good 2007–08 season. On 15 August 2007, he scored his first goal for the club in their Champions League 3rd qualifying round tie against Russian side Spartak Moscow.
Gordon Strachan usually employed Hartley, who had played as an attacking midfielder for Hearts, in a more defensive role. Hartley was one of Celtic’s key players in the defence of their league title at the end of the 2007–08 season in which Celtic had to win all seven of their remaining matches, two against arch-rivals Rangers. Playing in the centre of midfield with Barry Robson, Hartley helped Celtic to their third league title in a row.
Hartley featured less for Celtic during the 2008–09 season and was released by new manager Tony Mowbray on 1 July 2009.