Former Post Office sub-postmasters and mistresses will start giving evidence as part of a public inquiry on the Horizon scandal on Monday.
More than 700 branch managers were given criminal convictions when faulty accounting software made it look as though money was missing from their sites.
It has been described as the most widespread miscarriage of justice in UK history, with dozens of convictions overturned and many more in line for compensation.
What is this all about?
Between 2000 and 2014, the Post Office prosecuted 736 sub-postmasters and sub-postmistresses - an average of one a week - based on information from a recently installed computer system called Horizon.
Some went to prison following convictions for false accounting and theft, many were financially ruined and have described being shunned by their communities. Some have since died.
After 20 years, campaigners won a legal battle to have their cases reconsidered, after claiming that the computer system was flawed.
What was Horizon?
Horizon was introduced into the Post Office network from 1999. The system, developed by the Japanese company Fujitsu, was used for tasks such as transactions, accounting and stocktaking.
Sub-postmasters complained about bugs in the system after it reported shortfalls, some of which amounted to many thousands of pounds.
Some sub-postmasters attempted to plug the gap with their own money, even remortgaging their homes, in an (often fruitless) attempt to correct an error.
What was the effect on individuals?
Many former postmasters and postmistresses have described how the saga ruined their lives.
They had to cope with the long-term impact of a criminal conviction and imprisonment, some at a time when they had been pregnant or had young children.
Marriages broke down, and courts have heard how some families believe the stress led to health conditions, addiction and premature deaths.
"The past nine years have been hellish and a total nightmare. This conviction has been a cloud over my life," said former Oxfordshire sub-postmaster Vipinchandra Patel, whose name was cleared in 2020.
Seema Misra was pregnant with her second child when she was convicted of theft and sent to jail in 2010. She said that she had been "suffering" for 15 years as a result of the saga.
What was the turning point?
In December 2019, at the end of a long-running series of civil cases, the Post Office agreed to settle with 555 claimants.
It accepted it had previously "got things wrong in [its] dealings with a number of postmasters", and agreed to pay £58m in damages.
The claimants received a share of £12m, after legal fees were paid.
A few days later, a High Court judgement said that the Horizon system was not "remotely robust" for the first 10 years of its use, and still had problems after that.
The judge said the system contained "bugs, errors and defects", and that there was a "material risk" that shortfalls in branch accounts were caused by the system.
What has happened to the criminal convictions?
Following the High Court ruling, more cases were brought forward to the Criminal Cases Review Commission (CCRC), an independent body which investigates suspected miscarriages of justice.
In a series of rulings, the convictions of a total of 72 former postmasters have now been overturned, with more expected to go through the courts.
This included 39 postmasters' convictions being quashed in a single ruling at the Court of Appeal last April.
The judges determined that these 39 convictions were also "an affront to the public conscience".
That means the postmasters may pursue civil action against the Post Office for malicious prosecution, seeking significant sums in damages.
Will they receive compensation?
Those whose convictions, based on Horizon evidence, have been overturned will be in line for compensation, which the Post Office has promised as soon as possible.
It is inviting applications for interim compensation of up to £100,000, which will be funded by the government.
Final settlements will come via mediated agreements, or through claims in the civil courts.
What about other affected postmasters and postmistresses?
The Post Office has set up a separate historic shortfall scheme designed to repay those who lost out, but this has excluded those who were part of the High Court settlement. The government has now said those blocked from the scheme would be compensated "in parallel" to those who can use the shortfall programme.
More than 2,400 claims have been made to the scheme. Ministers said this was more than the Post Office expected and held the potential for the government having to step in to cover some of the cost.
The inquiry set up "to establish a clear account of the failings of the Horizon IT computer system, and assess whether lessons have been learnt at the Post Office" was given greater powers to investigate and call witnesses to give evidence. This followed pressure on the government after convictions started to be quashed.
It will take into account whether the Post Office knew about faults in the IT system and will also ask how staff shouldered the blame.
Has anyone been held accountable?
So far, nobody at the Post Office or Fujitsu has been held accountable, although the High Court judge said he would refer Fujitsu to the Director of Public Prosecutions for possible further action because he had "grave concerns" about the evidence of the company's employees.