2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/55328In this review I discuss some basic aspects of non-perturbative string theory. The topics include test of duality symmetries based on the analysis of the low energy effective action and the spectrum of BPS states, relationship between different duality symmetries, an introduction to M- and F-theories, black hole entropy in string theory, and Matrix theory.LaTeX file, 129 pages, 22 figures, Lectures given at Isaac Newton Institute and DAMTP, Cambridge, references added, typos correctedHigh Energy Physics - TheoryGeneral Relativity and Quantum CosmologyHigh Energy Physics - Phenomenologytext