This special issue of Journal of Physics A: Mathematical and General is dedicated to the memory of our friend and colleague Lochlainn O'Raifeartaigh, who was both a contributor to and referee for the journal. Lochlainn passed away at the age of 67 on 18 November 2000 after a short illness. He was born in Dublin in 1933, and most of his scientific career was centred around that city, where he obtained his first degrees at University College (BA in 1953 and MSc in Mathematical Physics in 1956), and spent from 1968 until his death as Senior Professor at the Dublin Institute for Advanced Studies. He obtained his doctorate from the University of Zurich (1960) under Walter Heitler. He also visited many institutions, notably Madras, IHES Bures, and Princeton, but it was during an extended stay at Syracuse University (1964-8) that he made the discovery that established his reputation. This result, which became known as O'Raifeartaigh's no-go theorem, showed that it was impossible to combine internal and relativistic symmetries other than in a trivial fashion, thus ending a widespread quest by the particle physics community to achieve this fusion. O'Raifeartaigh's prolific career in theoretical physics was manifested by many fundamental contributions to the application of symmetries in particle physics. In the 1970s he showed that the new supersymmetries could provide a mechanism (O'Raifeartaigh's mechanism) for circumventing his no-go theorem which had assumed only classical Lie group symmetries. In the 1980s he applied non-Abelian gauge theory to the analysis of magnetic monopoles. His interests encompassed the spin-statistics theorem, Kac-Moody and W-algebras, and included early contributions to the theory of non-invariance (dynamical) groups, among much else. This long and productive career in the application of symmetry methods to theoretical physics was acknowledged by the award of the Wigner Medal in August 2000. Lochlainn O'Raifeartaigh had a profound understanding of theoretical physics, and the even rarer ability to impart this understanding in class and in seminar. His remarkable talent for clear exposition ensured that he was always in demand as a lecturer, and his gentle manner and patience made him the first port of call for colleagues and students who wanted clarification of some aspect of their subject. He was a man of faith and humanity, devoted to his family and his country, and to the wider community of mankind beyond the borders of his profession. His passing is a great loss to Irish theoretical physics in particular, and to the world of physics in general.

Bruno Gruber, Vladimir I Man'ko and Allan I Solomon