2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/196143We comment on the present status, the concepts and their limitations, and the successes and open problems of the various approaches to a relativistic quantum theory of elementary particles, with a hindsight to questions concerning quantum gravity and string theory.To appear in: An Assessment of Current Paradigms in the Physics of Fundamental Phenomena, to be published by Springer Verlag (2006)High Energy Physics - Theorytext