They spend their day behind the high walls and secured doors of Her Majesty's establishments in a role often ignored outside of bad press or comedy stereotype.
So who becomes one of the prison service's "screws" and why?
It is stressful work in a stressful environment
"If we've managed to get through without them hurting themselves or anyone else," is how Prison Officer Sonia Leonard sums up a good day at work at HMP Wormwood Scrubs.
"When a man's locked away, he has to ask for everything. He can't go out and do anything, get a toilet roll, help himself," she explains.
"There's 10 of you, and 250 men, on one wing. Every one has a need and it's important to make time if they say 'I need to talk'."
Self harm, suicide risk and assaults are some of the serious problems the 53-year-old faces at the west London prison.
She has worked there for five years, as a standard grade officer, earning up to £26,000 a year. The role is hard, with high sick leave rates because of its physical nature.
Cutting is the most common form of self-harm among inmates, although none of the 250 men on Sonia's wing are currently logged in its record book.
The Scrubs also has one of the highest suicide rates among UK prisons and it is "more and more of a problem", says fellow officer John Hancock, 55.
He believes pressure on fewer staff make it harder to prevent.
Sonia hit the rock bottom of a ten-year career in 2004 when a 29-year-old lifer killed himself.
SUICIDES AT THE SCRUBS
2006 Jan-March: 1
Total 1996-present: 13
Howard League for Penal Reform
He had at least five years left to serve and as his personnel officer, she had to testify at his inquest.
"That's the worst thing I've ever had to do, my lowest point in my career, to go to an inquest and talk about somebody I have done a lot of work with," she says.
"When someone kills themselves it's a shock, a terrible experience. You have to go on because it's your job. There's other prisoners to talk to and it affects them."
How to deal with prisoner suicides, and its depressive impact among dozens of other inmates, is one of the aspects for which officers undergo weeks of classroom training.
They cover control and restraint of prisoners, using the radio, security, how to tackle assault and fighting, their own fitness and dealing with inmates' diversity, before taking up a post behind prison gates.
The service had 48,140 staff in 128 UK prisons at June 2005, with another 11 prisons in the private sector. Of those staff, 24,394 were prison officers.
They work a 39-hour week, in shifts around the clock, and the job entails many roles - gaoler, counsellor, restrainer.
Officers are more likely to take sick leave than those in other jobs
Each day has a regimented structure - from the counting of inmates at 0745, through meals, education, visits, work, association time - to lock-up at 2000.
It is stressful work in a stressful environment. In 2002, there were 2,692 assaults on prison staff. In 2004-5, 1,294 left the service, 112 of those on health grounds.
The service's Pay Review Body report for 2006 records that the physical nature of prison work means a higher level of sick leave.
But they face more than physical challenges in their daily work.
Both officers testify to the widespread drug use in prison and say that heroin, crack and cannabis have an impact on what they try to achieve with inmates.
The majority of prisoners have drug and alcohol abuse issues or are in for drug smuggling, partly because the Scrubs is situated close to Heathrow Airport and not far from Gatwick.
Drugs come over the prison wall day and night, says John. "If you're not on top of it it's a problem. There's a huge prison population that uses drugs. If they can't get hold of drugs, the bullying goes up."
They grapple with an "amazing" level of illiteracy among prisoners and communication is also an issue among the multi-cultural prison population - currently they lack enough Chinese translators.
And they say a lack of funding for staff, resources and courses harms prisoners' chances for rehabilitation and education.
In the 1990s and early 2000s, Wormwood Scrubs was the subject of a scandal over prisoner treatment. Of its then 400 officers, 27 were accused over ill-treatment of inmates. Three officers were jailed, but later acquitted, for violence.
The prison survived a damning report by the Chief Inspector of Prisons that gave it a year to improve or face closure.
Victorian, built 1875-1891
Category B prisoners
D: convicted and work
John says the period was "particularly difficult" with low morale. He would still welcome a full public inquiry into the episode and believes the case and report were partly politically motivated.
Morale has improved since then and both officers say a culture of mutual respect operates in the Scrubs; that there is a line they and prisoners approach, but do not cross.
"We don't get too friendly with them - that would be ridiculous - and they have respect for us," says John, 55.
"Most people who live and work in a prison know where that line is. If you make a promise to an inmate, you have to keep it."
'A soft word'
Given the level of strains and pressures they stand compared to a job outside prison walls, it is surprising to hear them describe their work in the definite, positive terms they use.
"Career-wise, its the best decision I ever made," says Sonia, who adds that a previous five years at Feltham Young Offenders Institution were "wonderful".
"When I applied and was accepted, it was a life-changing moment. I thought I would just have a job for the rest of my life, not a career, but this has opened up so many doors."
She applied to the service in her 40s after guarding prisoners in court: "I get on well with people. They came to court for a few hours, confided in me and I thought, there must be more I can do.
The Scrubs faced allegations of abuse in the 1990s
"A lot of them had personal troubles, abuse, abuse of alcohol or drugs. After 15-20 minutes they'd say 'thankyou' and I would think 'what have I done?' - it was that I'd listened.
"I'm no soft touch, but everybody - no matter what they've done - needs a soft word sometimes."
John says he was in the "right place at the right time" when he decided to join 18 years ago.
"I thought it would be a challenge to be a prison officer because so many different things crop up," he says.
"I was good in lead roles with other men. I've seen a little bit of life and the people you come across don't faze me."
Making prison a workplace has given them a unique perspective, however. Fresh from jury service on a fraud case, Sonia enjoyed the experience, but not the result.
"When we found them guilty, I felt awful. Because I knew where they were going, I knew what it involved, how they would feel."