This paper is based on few updated sections of the book published in 2004 by the Kluwer Aca-
demic Publishers, reprinted from the review paper by Giovannelli & Sabau-Graziati (2004).
Because of the impossibility of realizing a complete picture of the behaviour and physics govern-
ing the X-ray binary systems, I was obliged to select how to proceed, the subjects to be discussed
the more or the less, or those to be rejected. I have emphasized the important role played by the
magnetic field in these systems, of course with limitations due both to a reasonable length of this
paper and my knowledge.