The convictions of dozens of XO former sub-postmasters and sub-postmistresses have been overturned after the UK's most widespread miscarriage of justice.
It marks the latest stage of a computer scandal, and a long and complex legal battle, which could leave the Post Office and government with a huge compensation bill.
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.