Supporter and Rivalries

Chelsea have the fifth highest average all-time attendance in English football and regularly attract over 40,000 fans to Stamford Bridge; they were the fifth best-supported Premier League team in the 2009–10 season, with an average gate of 41,423. Chelsea's traditional fanbase comes from working-class parts of West London, such as Hammersmith and Battersea, from wealthier areas like Chelsea and Kensington, and from the Home Counties. In addition to the standard football chants, Chelsea fans sing songs like "Carefree", "Blue is the Colour", "We all follow the Chelsea" (to the tune of Land of Hope and Glory), "Ten Men Went to Mow", "Zigga Zagga", "Hello! Hello!" and the celebratory "Celery", with the latter often resulting in fans ritually throwing celery.

Chelsea do not have a traditional rivalry on the scale of the Merseyside derby or the North London derby; their West London derby with Fulham has not been as prominent over the years, with the two clubs often spending time in separate divisions. A 2004 survey by Planetfootball.com found that Chelsea fans consider their main rivalries to be with (in order): Arsenal, Tottenham Hotspur and Manchester United. Their rivalry with Tottenham Hotspur is said to have developed following the 1967 FA Cup Final, the first cup final held between two London clubs.

Additionally, a strong rivalry with Leeds United dates back to several heated and controversial matches in the 1960s and 1970s, particularly the FA Cup final in 1970. A more recent rivalry has grown with Liverpool following several clashes in cup competitions – particularly after what José Mourinho dubbed a "ghost goal" by Luis García in the UEFA Champions League 2004–05 semi-final, knocking them out of the competition.

During the 1970s and 1980s in particular, Chelsea supporters were long associated with football hooliganism. The club's "football firm", originally the Chelsea Shed Boys, now known as the Chelsea Headhunters, were nationally notorious for violent acts against hooligans from other teams, such as West Ham United's Inter City Firm and Millwall's Bushwackers, both during and after matches. The increase in hooliganism in the 1980s led chairman Ken Bates to propose erecting an electric fence to deter them from invading the pitch; the proposal was rejected by the GLC. Since the 1990s there has been a marked decline in crowd trouble at matches, as a result of stricter policing, CCTV in grounds and the advent of all-seater stadia.