Officially they are known rather grandly as Covert Human Intelligence Sources, but to the rest of us they are snouts, grasses or coppers' narks.
Especially to those who remember The Sweeney, the hard-hitting 1970s TV drama about the Flying Squad that often shone a light on the murky world of police informants.
People give police information for a variety of reasons - for money, to remove a rival, or to get a cut in their own sentence.
If an informant helps the conviction of others, police can give the judge a "text" - a letter - outlining the cooperation in expectation of a reduced sentence for their snout.
It's become big business for the police, though it's a subject they are reluctant to discuss in detail.
A recent Freedom of Information request revealed that UK forces spent £20m in five years on payments to informants.
Most informants have some criminal past and one drugs squad detective I knew viewed anyone he arrested as a potential snout.
Former Flying Squad detective Peter Kirkham told me: "In broad terms you set a thief to catch a thief, but it's usually more complicated than that. If you want to know about criminality you need to get information from someone involved in that criminality."
When Mr Kirkham began his detective career 30 years ago there were few checks on how officers used informants.
A detective kept the source secret and recorded no details of their meetings or payments. It was an unregulated system that was open to abuse, from both sides.
The Scotland Yard investigation into the notorious 1987 axe murder of private investigator Daniel Morgan was condemned years later when it was revealed that a detective had "coached" his informant.
In the 1990s the Yard was found to be using Jamaican Yardie gangsters as informants without realising they were carrying on their life of crime at the same time.
The Yardie case led to stringent oversight of the informant system, with new rules and a code of practice.
The latest Home Office guidelines set out strict procedures to be followed, especially on the use of juvenile informants.
Yes, police are allowed to use children as their narks, with certain caveats.
The Home Office guidelines say: "On no occasion should the use or conduct of a CHIS under 16 years of age be authorised to give information against their parents or any person who has parental responsibility for them."
Is there a line to be to be drawn anywhere over the use of informants? Can anyone, whatever their past, be hired by police to help them convict other criminals?
An informant in the Newcastle child sex case, a convicted child rapist, claimed during a pre-trial hearing that Northumbria Police had paid him £10,000 for information about the sexual exploitation of young girls.
He said part of his work included going to parties where police suspected girls would be intoxicated and sexually abused.
Peter Kirkham said: "I can't see any circumstances where somebody with his previous convictions would be approved as a participating informant to go to places where that sort of thing was happening.
"If the police had reasonable grounds to suspect what was going on they could not have allowed that to happen.
"It's not like me letting a villain steal a car for an armed robbery. In a case like that I would let the theft go ahead because it would lead me to a robbery where I could intervene and arrest more serious villains."
The Newcastle case is the biggest child sex investigation since the Rotherham and Rochdale grooming scandals and involved the trial of 25 suspects.
Former North West chief prosecutor Nazir Afzal, who led the Rochdale prosecution in 2012, said: "Putting a child sex offender in contact with children and other sex offenders is a line I would not cross.
"It is an ethical issue, but there is also the question of the danger to the children from someone who has already been convicted. My experience shows such men continue to offend.
"You need to be very wary. I can't think of anything that could be done to manage the very obvious risks."