2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/49676This is a set of lectures given at the 99' Cargese Summer School: Particle Physics : Ideas and Recent Developments. They contain a pedestrian exposition of recent theoretical progress in non-perturbative field theory and string theory based on ideas of duality.JHEP LateX, 56 pages, 13 eps figures;v2 References addedHigh Energy Physics - PhenomenologyGeneral Relativity and Quantum CosmologyHigh Energy Physics - Theorytext