The Soham murder trial renewed concerns about the validity of the system for vetting school staff in England.
Official guidance aims to minimise the number of checks being sought
How does it work?
The system was tightened up under the new Criminal Records Bureau, which began operating last year.
Under the old system, employers checked with the Department for Education whether someone's name was on the official lists of people deemed unsuitable to work with children.
These long-held lists are the Department for Education's List 99 and the Department of Health's equivalent.
The lists are cross-referenced so a check on one is a check on the other - so the procedure is usually referred to in education circles simply as "a List 99 check".
Employers dealing with a lot of staff had secure internet access so they could run List 99 checks themselves.
A crucial shortcoming of List 99 is that it only includes people already employed in an education-related capacity when a concern about them arose.
In addition to the lists, police forces would be asked to check whether someone had a criminal record - a local education authority, for instance, would typically have a designated liaison person.
This could take some time - months, even - so people could start work in the meantime provided they had a satisfactory List 99 check.
It goes without saying that other professional and character references also had to be taken up, and employment history checked - especially to account for any gaps.
The Criminal Records Bureau (CRB) was set up as a Home Office agency as a result of the Police Act 1997, though it did not begin work until 11 March 2002.
The idea was that the CRB, run by private consultancy Capita, would provide everyone with a quicker "one stop shop" for both List 99 and criminal record checks, issuing what it calls "disclosures".
The "standard disclosure" for people working with children and "vulnerable adults" checks the Police National Computer for any court convictions plus details of any police cautions, warnings or reprimands, and runs a List 99 check.
The "enhanced disclosure" for people "in sole charge of children or vulnerable adults" is the same as a standard disclosure but can also include locally-held police intelligence about an applicant, and obtaining that still takes time.
The CRB accesses the Police National Computer (PNC) which has details of offences from England and Wales, and copies of relevant Scottish criminal records.
The CRB says "plans are being made" to include criminal convictions from Northern Ireland, but at present these might not turn up in people's disclosures.
There is a big problem with the timeliness of the criminal records - with some taking much longer than the target seven days to get court details onto the system.
A leaked report from the police inspectorate says: "CRB checks can only be as good as the quality of data available on PNC and the failure of many forces to take the necessary action to reach nationally agreed targets leaves open the potential for known offenders or those suspected of serious offending to be overlooked during CRB checks."
Then there is the Protection of Children Act 1999.
The main purpose of that was to give access to the official blacklists to childcare organisations - those providing accommodation, social services or health care to children, or supervising children.
Plus, a huge range of other organisations such as Scouts and Guides, youth clubs, religious organisations and anyone running sporting and leisure activities for children, are now "encouraged" to join the checking system.
Although the checks are free to voluntary workers the administrative cost to organisations can be considerable.
Didn't the CRB have a rocky start?
When the CRB finally began work, new guidance was issued to education employers to make sure that all new staff were checked against List 99 and police records before starting work.
It was immediately apparent that it simply couldn't cope with the volume of requests.
A backlog of applications for standard and enhanced disclosures rapidly built up.
Teacher recruitment agencies were saying they couldn't get people into schools because checks were taking many weeks.
Apart from the sheer volume of requests, it is also thought the bureau had expected most requests to come by phone whereas they were in writing.
The Home Office said that about half the forms that had been received had mistakes on them.
So in May 2002 the Department for Education told employers to adopt an "interim arrangement" - in effect, the old system but done through the CRB.
The CRB trawled its backlog of applications for those from classroom staff, and ran a List 99 check on them.
If that was clear, they could be appointed provisionally, pending completion of the full police check.
A letter the department sent to education authorities and teacher agencies made it clear this exemption applied only to "teachers and other classroom staff" - not contractors, ancillary staff such as caretakers, or school governors.
Teachers and teaching assistants were put into schools on the basis only of the List 99 checks.
Following the murders of 10 year olds Jessica Chapman and Holly Wells, the department scrapped this policy and insisted all new staff must obtain a full disclosure from the CRB before starting work.
Hence the panic, with the CRB bringing in extra staff and working flat out to try to complete checks on teachers before the start of term in the autumn of 2002.
When it failed, and schools had to turn children away because they did not have enough fully-vetted staff, the then education secretary, Estelle Morris, changed tack again - giving head teachers discretion to let people start work after List 99 checks (now being done by her department, as they used to be), with a full disclosure to follow later.
So is that the current system?
Yes - except there was some more Department for Education and Skills guidance in December 2002, tellingly entitled "managing the demand for disclosures".
It says for example that some people had been seeking repeat disclosures on prospective employees who were changing jobs.
So the advice was not to bother unless there had been an employment break of more than three months.
The guidance stresses the test of "posts which regularly involve caring for, training, supervising or being in sole charge of young people".
It says: "It is unlikely that cooks, cleaners, administrative staff and technicians, for example, would qualify for enhanced disclosures."
But a spokesperson said: "We take the view that caretakers are likely to fall into this category and so will qualify for an enhanced disclosure."
The department says schools or education authorities should not seek retrospective checks on existing staff or governors unless they have grounds for concern about someone's suitability to work with children.
What about Scotland?
As with most things the system in Scotland is slightly different.
The Scottish Criminal Record Office (SCRO) provides a broadly equivalent service, called Disclosure Scotland, which also started work in 2002.
As elsewhere, all teachers in Scotland's local authority schools have to be registered with the General Teaching Council Scotland and about 90% of independent school staff also are registered.
Prior to registration the council carries out a Scottish Criminal Records Office check.
Local authorities run similar checks on support staff in schools.
Perhaps surprisingly, the Scottish Executive only then set up a blacklist akin to List 99.
And Northern Ireland?
Since 1990 the Department of Education for Northern Ireland has required a criminal record check on anyone before they are appointed to a position giving "substantial access to children".
This applies to employees and volunteers, whether full-time or part-time.
The list of posts covered is extensive: all professional, ancillary, administrative and clerical staff employed by all state and independent schools and further education institutions, guidance centres, education and library boards, the youth and music services - even home tuition teachers, drivers and school crossing patrol staff.
In addition the department records cases of misconduct by teachers, which takes account of not only court convictions but dismissals and even press reports.
It considers "withdrawing recognition" from the teacher concerned.
When someone seeks a teaching post the department checks this record, and the lists maintained elsewhere in the UK.
What about teachers from overseas?
They are not covered by the CRB so there is no point employers asking for disclosures.
It offers guidance about the availability of criminal record checks in foreign countries, which is patchy.
The Department for Education says it is up to schools or education authorities to contact their local police, who would then contact their opposite numbers in the teacher's home country to seek a criminal records check.
In some countries, individuals can obtain a "certificate of good conduct" or the equivalent - but practice varies considerably.
In the Republic of Ireland, there are no arrangements for making criminal records checks in connection with child protection - except for fostering and adoption. Nor are there good conduct certificates.
Are university staff checked?
No. In education, the rules apply to those working with "children" - that is, aged 18 and under.
The government decided there was no need to extend the vetting process to the higher education sector - even though it was pointed out that increasing numbers of teenagers go to universities for such things as summer schools.
For that matter, anyone can set up as a private tutor and invite children into their home on a one-to-one basis, without any regulation.