Thousands of Palestinian political prisoners are being held by Israel - and many thousands more have passed through Israeli jails during nearly three years of the current Palestinian intifada.
The fate of these detainees is not mentioned in the US-backed peace plan for the Middle East, known as the roadmap.
But it remains a significant bone of contention that from time to time has the capacity to bring peace negotiations to a screeching stop.
At the time of writing, about 5,600 Palestinians are in Israeli custody for political reasons - there are also a few hundred common criminals.
ISRAEL'S PALESTINIAN INMATES
5,600 Palestinian security inmates
2,420 serving sentences
2,650 awaiting trial
530 'administrative detainees'
1,500+ with 'blood on hands'
The batch of about 350 prisoners released as a "goodwill gesture" by Israel on 6 August gives a good indication of the kind of activity many Palestinians are jailed for.
Membership of Hamas, Islamic Jihad or other Palestinian militant organisations, stone-throwing, assisting militant organisations or "wanted" Palestinians, firearms and explosives offences.
A category of prisoners not released on Wednesday was those considered to have "blood on their hands", Israel's term for people allegedly involved in fatal attacks against Israelis.
The proportion of such prisoners in the larger population is a matter of speculation.
Israel's prison service, whose jails hold about half the Palestinians currently in custody, says 1,500 of its charges come under this category - either because they have been convicted or are awaiting trial for carrying out terrorist acts.
But the army (IDF) which holds the other half in detention, was not able to furnish the BBC with detailed statistics.
Other categories of prisoners are easier to establish: about 75 are women, and 360 boys under the age of 18 who - controversially - are sometimes kept among the adult population.
The prison service says that of its 2,700 security prisoners, about 1,250 are being held on remand and 1,450 have been convicted.
The IDF holds 2,900 prisoners, including 970 who have been convicted and 1,400 on remand or arrested on judges' orders.
There are also at least 530 "administrative detainees" in IDF custody, who are held without charge or trial for renewable six-month terms.
Administrative detention is a system revived from British Mandate times (before Israel was established in 1948) and has drawn harsh condemnation from human rights watchdogs who say it is illegal and arbitrarily applied.
Unlike their best-known inmate - Marwan al-Barghouti - most Palestinians face military trials
All such detainees should be released immediately or charged with specific crimes and put on trial, rights groups say.
But the Palestinian lawyers' NGO al-Haq says even when detainees are tried, the Israeli military authorities responsible for administering justice in the occupied territories overlook the standards of fair trials on several levels.
Alleged violations include depriving detainees of proper representation by counsels, the use of confidential evidence not disclosed to counsels or defendants, and giving evidence briefings in Hebrew without access to a translation.
The IDF denies the specific allegations and says it does everything to ensure fair trials, though it is hampered by the large numbers of people it says it must detain to ensure the safety of Israeli citizens.
Living conditions for prisoners are generally described by observers as extremely harsh and difficult.
Human Rights Watch says the 4,500 Palestinians arrested in the massive military sweep through the West Bank in March-April 2002 were subjected to widespread ill-treatment such as kicking, beatings, squalid conditions and deprivation of food and drink.
Overcrowding and very austere conditions appear to be common complaints in the main prisons, and al-Haq says prisoners with chronic medical conditions receive far from satisfactory care and treatment.
Ketziot prison in the Negev desert is hard to visit for Palestinian relatives
A senior Israeli military source admitted to BBC News Online that overcrowding was a problem, but categorically denied any maltreatment.
Prisoners received exactly the same medical attention as Israeli soldiers, and Ketziot prison - for example - has just had $5m spent on it to deal with some of the accommodation problems, the source said.
Visits by families are hampered by travel restrictions imposed on all Palestinians, the prisoners' support group Addameer says, with inhabitants of the West Bank towns of Nablus and Jenin denied access to jailed relatives since May 2000 - that is four months before the intifada even started.
But the most controversial issue remains Israel's reported use of physical and psychological pressure during interrogations of prisoners, a practice internationally condemned as torture by the United Nations and human rights groups.
Israel's High Court outlawed the use of what was termed "moderate physical pressure" of prisoners in September 1999, but the Public Committee Against Torture in Israel says interrogators have gradually reverted to techniques that amount to torture.
According to human rights groups, these include beating, leaving prisoners in uncomfortable postures, interrogation sessions which last 24 or 48 hours, depriving prisoners of sleep, depriving them of human dignity and making threats against the lives and property of their relatives.
But Israel defends its interrogation techniques - as it defends the whole edifice of legal and penal mechanisms it uses in relation to the Palestinians - as a legitimate way of combating terrorism faced by its citizens.