The bulk of the correspondence consists of letters received, covering the period 1920-2004. This includes handwritten draft responses by Rotblat, some photocopies, as well as typed outgoing copy letters. Although mostly letters received, the term correspondence has been used for ease of reference. All series of correspondence include personal matters discussed in the same letter as professional matters. It is therefore not possible to separate personal and professional. Personal does not include private, immediate family correspondents, which form part of RTBT Series 1, (formerly Section A). Correspondents include individuals as well as societies, organisations and companies. As the original order is no longer discernible, correspondents are arranged alphabetically by surname and organisation name for ease of reference. The papers provide evidence of Rotblat's relationships and professional discussions with peers, as well as how Rotblat helped friends, family and colleagues. There is reference throughout to Pugwash (RTBT Series 5). His post-retirement (after retirement from St Bartholomew's Hospital) correspondence shows an increase in correspondence with national and international organisations as well as his broadening and widening circle of contacts. The series includes Polish letters and papers: where correspondents can be identified, they have been placed at the appropriate place in the series. Where it has been difficult to identify correspondents a separate sub-series has been created for Polish correspondence.