The admission by the director general of the Prison Service that he does not know how many inmates are on the run from open jails has put the spotlight on such facilities.
North Sea Camp's farm supplies other prisons with food
About 5,000 inmates out of a prison population of almost 80,000 in England and Wales are currently housed in just over a dozen open prisons.
The open jails include Ford in West Sussex, North Sea Camp in Lincolnshire, Hollesley Bay in Suffolk, and Leyhill in Gloucestershire.
The Home Office says open prisons are the "most effective" way of ensuring inmates are ready to rejoin the community before their release.
But people sent to an open jail need to be risk assessed and categorised as a low-risk to the public before being transferred from higher security facilities.
Classed as Category D, open prisons have a more relaxed security regime and are generally used to prepare inmates for their release.
White-collar criminals doing time for crimes such fraud and deception traditionally spend much of their sentence in open prisons.
Inmates convicted of serious crimes including murder and rape can be placed in open prisons towards the end of their sentences if assessed as low-risk.
Prisoners are able to wander around freely but must show up for daily roll calls. They can be sent back to more secure facilities for disobeying regulations.
Conditions in the open prison are a step up from a prisoner's time behind bars in high and medium security facilities.
Inmates are often housed in single rooms with their own keys. The rooms can contain a small TV as well as a bed, locker, table and chair.
A wider range of catering options is also on offer in open prisons.
There are specialist services providing help with job preparation and finding accommodation, as well as debt, drug and alcohol counselling.
Inmates can do academic classes, business studies or technical training, and there is paid and voluntary work in the community for long-term prisoners.
On-site work includes horticulture and vocational activities in workshops.
The hundreds of acres of land around North Sea Camp, for example, is one of the Prison Service's largest production farms, supplying other jails with vegetables, corn and meat.
Hollesley Bay has a large stud of Suffolk Punch horses which are shown at local county and national shows.
The government came under fire last year when it revealed in a written answer that some 357 prisoners who have absconded from open prisons since 1997 remain at large.
The National Association of Probation Officers has warned higher risk prisoners were now being held in open units following changes introduced in October 2006 to deal with the overcrowding crisis.
But Prison Service director general Phil Wheatley insisted inmates being held in open prisons did not pose a greater risk to the public than before.
Officials have been working to ensure more non-violent offenders on short sentences were held in open conditions, freeing up places in local jails for more dangerous prisoners, he said.
Some prison reformers, meanwhile, have expressed concerns that some inmates have been unable to take advantage of support programmes because they have been moved around between facilities.