What are the main allegations?
In January 2011, environmental campaigners discovered that Mark Kennedy, an undercover police officer, had spent years pretending to be one of them.
His true identity emerged after activists in Nottingham were accused of planning to occupy a strategically vital power station. He admitted his role in the affair and offered to give evidence in their defence - leading to the overturning of criminal convictions and the collapse of further prosecutions.
That was just the start of it. Mr Kennedy had had a series of relationships with women.
One woman, Kate Wilson, spent two years in a relationship with the officer who had a parallel real life elsewhere.
Another, known only as Lisa for legal reasons, was with Mr Kennedy for much longer.
"It was exactly 10 years ago that I discovered that my partner of six years was actually a policeman," says Lisa. "He was a fictional character - his identity was fabricated and he was put into my life to deceive me, by his employer, who knew that one day they would remove him."
Soon, another former officer, Peter Francis, revealed himself. Mr Francis had not been in any relationships - but he'd been deployed against justice campaigns.
Some officers appear to have gathered intelligence on Labour MPs including Peter Hain, once a leader of the UK's anti-apartheid movement, and Jack Straw, who went on to become the home secretary.
Many officers deployed by the SDS created their undercover personas after being instructed to research records of real children who had died young - and then "resurrect" that identity for their own use.
Who are the complainants?
There are 200 "core participants" in the inquiry. They include:
- Women who had relationships
- Justice campaigns - including families of victims of racist murders
- Political activists in left-wing groups and anti-capitalist movements
- Labour politicians
- Environmental protesters - including many people who have spent decades warning of climate change
- Animal rights activists
- Trade unionists who say they were blacklisted from taking work based on their political views
What do the complainants want?
Many want to simply know why their lives were spied on and what information the police gathered on them.
It's been suggested that the undercover units gathered information on up to 1,000 different groups over 40 years - there are millions of pages in the undercover archives.
They also want to know the identities of the officers. As things stand, the vast majority are not going to be revealed in public. Some 69 fake names used by officers
have been published - but many more remain unknown.
Mr Francis, the former officer who accused his bosses of running an operation against the Lawrence family, also wants recognition for the mental scarring that many officers live with - and transparency about who at the top of policing knew what was happening.
Have the police admitted any wrongdoing so far?
In 2015 the Metropolitan Police made an
"unreserved apology" to women whom it admitted had been deceived into relationships that should never had happened - and paid out compensation.
Why has the inquiry been so delayed?
An enormous team of lawyers has spent five years working out how it can actually hear evidence because of the complicated and often competing issues relating to secrecy.
To date, it has cost almost £30m just to get to the starting point of hearing evidence.
After the November 2020 opening, it will run for at least three years as it works its way chronologically through the operations that took place.