  These lectures intend to give a pedagogical introduction into some of the
developments in string theory during the last years. They include perturbative
T-duality and non perturbative S- and U-dualities, their unavoidable demand for
D-branes, an example of enhanced gauge symmetry at fixed points of the
T-duality group, a review of classical solitonic solutions in general
relativity, gauge theories and tendimensional supergravity, a discussion of
their BPS nature, Polchinski's observations that allow to view D-branes as RR
charged states in the non perturbative string spectrum, the application of all
this to the computation of the black hole entropy and Hawking radiation and
finally a brief survey of how everything fits together in M-theory.
