Informers would testify against their former comrades
Two UVF members from north Belfast, David and Robert Stewart, have pledged to give evidence against nine men in what promises to be the biggest "supergrass" trial in a generation. BBC News Online looks back at previous cases.
The term "supergrass" rose to prominence in Northern Ireland during the early 1980s.
They were informers, or suspects, prepared to testify in court against their alleged ex-comrades in return for rewards such as immunity from prosecution, lenient sentences, and new identities.
The trials in Northern Ireland in which they featured were some of the largest ever conducted in the UK, with the evidence of a single witness being enough to convict dozens of accused.
The practice became discredited when doubts began to be raised as to whether they were reliable witnesses.
In one 1983 supergrass trial, 22 IRA members were given jail terms totalling 4,000 years.
They were convicted largely on the evidence of Christopher Black, who was granted immunity from prosecution and is thought to have fled abroad after the trial.
The term grass comes from the lexicon of the London criminal underworld of the 1930s, to refer to an informer.
The first supergrass was Bertie Smalls in 1973.
The leading member of a gang of London bank robbers after being arrested he offered to help the police by naming his accomplices in return for his liberty. His evidence resulted in 16 convictions.
More recently Darren Mathurin became Britain's first black supergrass with the second trial he testified at ending recently at the Old Bailey.
The 29-year-old drug dealer, with the street name Spider, from the Stonebridge estate in London, has testified at two trials and has had a murder tariff reduced from 22 to eight years.
Eighteen of those convicted on Black's evidence had their convictions quashed three years later.
The IRA trial followed another case involving members of the loyalist Ulster Volunteer Force.
Key evidence in that trial came from Joseph Bennett, who was also granted immunity, which led to 14 men being imprisoned - two for life terms.
Christopher Black and Joseph Bennett were among about 30 former paramilitaries who turned informant and their evidence led to more than 300 convictions.
They also included Raymond Gilmour from Londonderry, who was in the INLA and the IRA. When he decided to testify it led to the arrest of about 100 republicans in the city, 35 of whom were charged with terrorist offences.
The case collapsed when the then Lord Chief Justice dismissed his evidence as being "unworthy of belief".
Three years ago he appealed to be allowed to return to Derry
But for supergrasses there can be little chance of going back, those who work for the authorities or are suspected of being agents still face extreme dangers.
In an interview with the Sunday Tribune in 2009 the Real IRA named five men they wanted to kill, two of them were Christopher Black and Raymond Gilmour.
The last supergrass trial was in 1985, when 25 members of the INLA were jailed on the evidence of Harry Kirkpatrick.
By December 1986, 24 of them would have their convictions overturned.
For the security services the trials removed suspected terrorists from the community, albeit in most cases only for a short time..
Supergrasses face making new lives after testifying
But they raised serious concerns about how justice in Northern Ireland operated, with the Earl of Longford commenting in one debate in the House of Lords: "You cannot cast out Beelzebub by Beelzebub."
However, a change in the law in 2005 laid the way for the possibility of new supergrass trials.
Under the Serious Organised Crime and Police Act, an accused can enter into a written agreement indicating that he will help the prosecution by giving evidence against other criminals with whom he was involved.
Where this happens the court may take this into account when passing sentence.
The Court of Appeal for England and Wales has noted that such a reduction in sentence is the only incentive that can be offered to encourage the provision of information about serious criminals.
Furthermore the court noted that the legislation in effect accepts that "this is a price worth paying to achieve the overwhelming and recurring public interest that major criminals, in particular, should be caught and prosecuted to conviction".