  A review of basic properties of Be/X-ray binaries is presented. These systems
(called also hard X-ray transients), which form the most numerous class of
massive X-ray binaries in the Galaxy, are composed of Be stars and neutron
stars (X-ray pulsars) on wide (orbital periods 17-263 d), usually eccentric
(eccentricities 0.1-0.9) orbits. The systems contain two quasi-Keplerian (ratio
of radial to orbital velocity components less than 0.01) discs: decretion disc
around Be star and accretion disc around neutron star. Both discs are
temporary: decretion disc disperses and refills on time scales of years
(dynamical evolution of the disc, formerly known as the "activity of a Be
star"), while accretion disc disperses and refills on time scales of weeks to
months (which is related to the orbital motion on an eccentric orbit and, on
some occasions, also to the major instabilities of the other disc). Accretion
disc might be absent over a longer period of time (years), if the other disc is
very weak or absent. The X-ray emission of Be/X-ray binaries has distinctly
transient nature and is controlled by the centrifugal gate mechanism, which, in
turn, is operated both by the periastron passages (Type I bursts) and by the
dynamical evolution of the decretion disc (both types of bursts). The X-ray
pulsars in these systems rotate at equilibrium periods (with the possible
exception of the slowest pulsars). Be/X-ray binaries are excellent laboratories
for investigation of both the evolutionary processes in the two discs and of
the evolution of the neutron star rotation.
