The Green Brigade is a self-styled "ultras" group of Celtic fans known for banner displays and chanting.
The group, which occupies part of the north curve of Celtic Park, organises choreographed displays known as tifos and encourages singing during matches.
In recent years, the Green Brigade has been increasingly vocal on non-football related issues.
Some members have had their season tickets withdrawn by Celtic after a "serious escalation" in unacceptable behaviour.
Who are the Green Brigade?
The Green Brigade is made up of about 1,000 Celtic fans - 250 of whom are officially registered with Celtic as being part of the Green Brigade Supporters' Club.
This is how Celtic will know which season tickets to suspend during the newly-announced indefinite ban.
The group has season tickets in the Rail Seating Section- essentially a standing area - of the north curve of Celtic Park.
The Green Brigade was formed in 2006 in an attempt to improve the atmosphere during match-days. In addition to the choreographed banner displays in the stands (known as tifos), the group often bring drums, encourage singing and use pyrotechnics.
According to the group's own website, the Green Brigade is "notoriously difficult for fans to join".
Members often keep their identities hidden, with their faces being blurred in photos posted online. In the same vein, the group recently said that no members have or will speak to the "mainstream British press" about "the current situation" - referencing the withdrawal of tickets.
They have described the move by Celtic as shaming the club by "attempting to censor and sanction Palestinian solidarity".
The group also describe themselves as anti-fascist and left-wing.
They are often associated with supporting causes such as Irish Republicanism and the Palestinian cause.
Other Scottish football clubs also have their own "ultra" groups.
Rangers have groups such as the Union Bears while in Edinburgh you'll find the Gorgie Ultras, a group of Hearts' fans, and Block Seven made up of Hibernian supporters.
Not all Celtic fans, or season ticket holders, share the views of the Green Brigade.
Why have they been banned?
Some members of the Green Brigade have had their season tickets withdrawn indefinitely, adding to an existing ban on them purchasing tickets for away games, which essentially means they have been banned from attending Celtic matches.
The move follows the Green Brigade recently defying pleas by Celtic not to bring Palestinian flags to matches.
But the club said the indefinite ban had been sparked by a series of incidents at other games over an extended period of time.
The widespread and unsafe use of pyrotechnics at Feyenoord on 19 September
Rushing turnstiles and forcing open fire exits at Fir Park, Motherwell, on 30 September
Illegally gained access to Celtic Park in advance of the match against Lazio on 4 October to bring in an unauthorised banner
Violent and intimidating behaviour towards stewards at Easter Road, Edinburgh, on 28 October
This is not the first time the Green Brigade have been banned from Celtic Park.
In 2013, the club temporarily closed Section 111 in the football stadium, which was home to the Green Brigade, after safety warnings were repeatedly ignored.
This ban came around the time Celtic fans caused £10,000 worth of damage to the away end at Fir Park following a match against Motherwell.
A total of 128 members of the Green Brigade who were at the away game were banned from attending matches, while those not in attendance had their seats in Celtic Park relocated, for a total of nine months.
They were welcomed back during the 2014/15 football season after discussions with the club.
In 2010, the group made headlines after protesting against players wearing the poppy.
Members displayed a banner which read: "Your deeds would shame all the devils in Hell. Ireland, Iraq, Afghanistan. No bloodstained poppy on our Hoops."
Celtic issued an apology for any offence caused.
The club was also fined £13,000 by Uefa after fans displayed an offensive anti-monarchy banner following the death of Queen Elizabeth II.
Many Celtic fans have long had an affiliation with the Palestinian cause, with Uefa fining the club in 2014 after fans waved Palestinian flags during a match against Iceland's KR Reykjavik.
The Green Brigade chose to display the flag once again during their team's 2016 Champions League qualifier against Israeli side, Hapoel Beer-Sheva - a move which landed the club a £8,600 fine. Celtic later closed the area of the stadium occupied by the Green Brigade for a 2017 Champions League qualifier against Rosenborg.
Following the Uefa fine, the Green Brigade launched a campaign aiming to "match the fine for Palestine", and raised a total of £176,000 which was donated to the Medical Aid for Palestine charity and the Lajee Centre in the Aida Refugee Camp in the West Bank.
Their relationship with the Aida Refugee Camp continues, with the group helping to fund a football academy within the camp.
Green Brigade members also pride themselves in putting Celtic at the heart of the community in Glasgow's East End.
For the past 10 years the group, have organised an annual foodbank collection which this year raised £12,293 to be donated to six different foodbanks.