The group of known X-ray sources associated with a Be star in a binary system consists of about 40 objects. They are the most abundant type of massive X-ray binary in the Galaxy. In this contribution we present an up-to-date picture of the main physical characteristics of these objects, including their evolutionary history, mass transfer mechanism, an interpretation of their behaviour in the orbital versus spin period diagram and the use of Be/X-ray sources as galactic tracers.