The almost irresistible beauty of string theory has seduced many theoretical physicists in recent years. Even hardened men have been swept away by what they can already see and by the promise of even more. It would appear fair to say that it is not yet clear what form the theory will finally take and in what precise way it will relate to the physical world. However, it would seem equally fair to state that, most likely, strings are here to stay and will playa profound and central role in our conception of the universe. There is therefore a pressing need to provide both practicing physicists and advanced students with ways to master quickly, but soundly, the basic principles of the theory. The present volume is a step in that direction. It contains a lucid presentation of the basic principles of string theory in forms which may survive future developments. The book is an outgrowth of lectures given by Lars Brink and Marc Henneaux at the Centro de Estudios Cientificos de Santiago. The lectures covered in a self-contained manner different but complementary aspects of the foundations of string theory.