  In these introductory lectures we summarize some basic facts and techniques
about perturbative string theory (sections 1 to 6). These are further developed
(sections 7 and 8) for describing string propagation in the presence of
gravitational or gauge fields. We also remind some solutions of the string
equations of motion, which correspond to remarkable (NS or D) brane
configurations.
  A part II by Emilian Dudas will be devoted to orientifold constructions and
applications to string model building.
