For most of us, they are the law enforcers you don't even know exist.
But for those involved in violent criminality, it will be the Metropolitan Police's elite gang units who confront them head on.
I was given the eye-opening experience of spending three nights with a team of officers on the front line of the battle to rid our nation's capital of its unenviable record of violent crime.
During the time of filming, there was an average of one knife murder every other day in London.
Myself and Sky's Home Affairs Correspondent Mark White spent our time with PC Joe Broadfoot and his partner in fighting crime, PC Darren Porritt, from Lambeth's gangs unit.
Two very nice guys, but you don't want to get on the wrong side of them. Trust me.
They wear plain clothes and drive an unmarked car - but every gang member in south London knows exactly who they are.
Because of the nature of their job, this specialist team are a little more "robust" than your average bobby on the beat. Every suspect they stop potentially carries a weapon.
If the suspect resists arrest, which many of their targets do, they are dealt with by force.
In one episode, the gang unit were called to Streatham in south London to reports of a knife attack. They prowled a housing estate like sharks circle their prey.
When they spotted their suspect, who they believed was carrying a knife, they moved in fast. The suspect, a young black male, didn't want to be cuffed and began abusing the officers.
One of the officers held a CS gas canister to his face and screamed: "Your choice!"
:: Stop and search key to London's battle with knife crime menace
The suspect kept resisting and kept abusing. The CS gas was sprayed and the suspect wrestled to the floor. He was searched but no knife found.
While giving him what they call "CS aftercare", the suspect screams: "I should spit in your f***ing face you ****."
The suspect then turned on me. "I'll kick your face in," he shouted while being led away into the van.
Police tell us his mood changed when he got to the police station. He apologised for his behaviour and was ruled out of the investigation and released without charge.
"He matched the description of the suspect and for all we knew he was carrying a knife," PC Broadfoot said.
"And his behaviour didn't change our opinion. We had no choice but to deal with him forcefully. I'm afraid it's the reality of what we have to deal with."
This was an incident which escalated from a routine stop and search to quite a violent arrest, and to the many passers-by, it will have been understandably alarming.
These are the types of incident which are captured on mobile phones and posted on social media. And taken in isolation, they can look like over-zealous policing.
But I was there throughout. I saw a situation which spiralled out of control because of one man's distrust and disrespect for the police.
But then again, who can blame him? Black people are four times more likely to face a stop and search, according to a police watchdog.
For the gang unit, it was just another day at the office.
If you see them at work, you probably won't notice them. But the gangs certainly will.