Traditionally supporters come from Middlesbrough itself and towns in the immediate area.
Middlesbrough have one of the highest proportions in Britain of locally-born season ticket holders at 80%, and one of the highest proportions of female fans at 20%.
A survey at the start of the 2007–08 season found Middlesbrough supporters were the seventh loudest set of fans in the Premier League.
Middlesbrough Official Supporters Club, which features their own team in the local football league, has links with supporters' clubs across the globe.
The largest supporters' clubs include the Official Supporters' Club, the Middlesbrough Disabled Supporters' Association, Yarm Reds, and Middlesbrough Supporters South.
Middlesbrough supporters' main rivals are Newcastle United (with whom they contest the Tyne-Tees derby), Sunderland, and Leeds United, a fact confirmed by planetfootball.com's 2004 survey, where Newcastle and Sunderland fans also considered Middlesbrough to be amongst their top three rivalries.
The nickname Smoggies was first used as a derogatory term by opposing supporters, it relates to the industrial air pollution (smog) that has been ever present since the Industrial Revolution.
But was later used by Middlesbrough fans in a somewhat self-deprecating manner before finally being adopted as a badge of pride by supporters of the club.
An example of this can be seen on the banners carried to away games stating "Smoggies on Tour".
Middlesbrough fans were notably praised by UEFA Chief Executive Lars-Christer Olsson after their behaviour during the 2005–06 UEFA Cup campaign. He commended that:
You have the satisfaction of knowing that, although your team did not win the game, your supporters present in Eindhoven proved to the world that football fans can turn a match into a friendly, violence-free celebration.
Middlesbrough fans had also been praised by Cleveland Police force for their behaviour in previous rounds, particularly in the light of aggravation prior to and during the match at Roma.