Records of prisoners of war (POWs) were compiled by each country and are now held centrally by the Archives Division and Research Service of the International Committee of the Red Cross in Switzerland. Because of their personal nature, these records are not available to the general public. Prisoner of War Records for the First World War have been digitised.
Some details about those who died in captivity during both World Wars will be held by the Commonwealth War Graves Commission and is available through their Debt of Honour database.
Any official documentation relating to British POWs is held by The National Archives (TNA). More information about these records can be found in the research guides British Prisoners of War, c1760 – 1919 and British Prisoners of War, 1939 – 1953 .
The main First World War official sources are the interviews and reports provided by repatriated or escaped prisoners in record class WO 161. These can be viewed online on a pay-per-view basis. Although more than 3,000 individuals are represented, this is only a very small percentage of those who were held captive. There is also a small possibility that you may find correspondence or reports on prisoners of war in FO 383.
TNA holds a vast amount of material for Second World War prisoners of war. Japanese prisoner of war index cards can be found in WO 345. TNA also holds liberated prisoner of war interrogation questionnaires in WO 344. These were the debriefing reports given by prisoners released from German or Japanese captivity at the end of the Second World War.
The Children of Far East Prisoners of War website contains useful information, including a copy of the front page of the prisoner's liberation questionnaire, if one was completed.
Sources at IWM include personal papers and diaries, autobiographies, camp journals, photographs, artworks and recorded interviews, although inevitably some locations and periods are better documented than others. These can be extremely helpful for contextualising life as a prisoner of war.