  Quantitative spectroscopy is a powerful technique from which we can extract
information about the physical properties and surface chemical composition of
stars. In this chapter, I guide the reader through the main ideas required to
get initiated in the learning process to become an expert in the application of
state-of-the-art quantitative spectroscopic techniques to the study of massive
OB stars.
  NB: This chapter is intended to serve to young students as a first approach
to a field which has attracted my attention during the last 20 years. I should
note that, despite its importance, at present, the number of real experts in
the field around the world is limited to less than 50 people, and about one
third of them are close to retirement. Hence, I consider that this is a good
moment to write a summary text on the subject to serve as guideline for the
next generations of students interested in joining the massive star crew. If
you are one of them, please, use this chapter as a first working notebook. Do
not stop here. Dig also, for further details, into the literature I quote along
the text. And, once there, dig even deeper to find all the original sources
explaining in more detail the physical and technical concepts that are
presently incorporated into our modern (almost) automatized tools.
